<input>: The HTML Input element

How an <input> works varies considerably depending on the value of its type attribute, hence the different types are covered in their own separate reference pages. If this attribute is not specified, the default type adopted is text.

The available types are as follows:

<input type="button" name="button" value="Button" />

<input type="checkbox" name="checkbox"/>

<input type="color" name="color"/>

<input type="date" name="date"/>

<input type="datetime-local" name="datetime-local"/>

<input type="email" name="email"/>

<input type="file" accept="image/*, text/*" name="file"/>

<input id="userId" name="userId" type="hidden" value="abc123">

<input type="image" name="image" src="" alt="image input"/>

<input type="month" name="month"/>

<input type="number" name="number"/>

<input type="password" name="password"/>

<input type="radio" name="radio"/>

<input type="range" name="range" min="0" max="25"/>

<input type="reset" name="reset"/>

<input type="search" name="search"/>

<input type="submit" name="submit"/>

<input type="tel" name="tel"/>

<input type="text" name="text"/>

<input type="time" name="time"/>

<input type="url" name="url"/>

<input type="week" name="week"/>

<input type="datetime" name="datetime"/>

The <input> element is so powerful because of its attributes; the type attribute, described with examples above, being the most important. Since every <input> element, regardless of type, is based on the HTMLInputElement interface, they technically share the exact same set of attributes. However, in reality, most attributes have an effect on only a specific subset of input types. In addition, the way some attributes impact an input depends on the input type, impacting different input types in different ways.

This section provides a table listing all the attributes with a brief description. This table is followed by a list describing each attribute in greater detail, along with which input types they are associated with. Those that are common to most or all input types are defined in greater detail below. Attributes that are unique to particular input types—or attributes which are common to all input types but have special behaviors when used on a given input type—are instead documented on those types' pages.

Attributes for the <input> element include the global HTML attributes and additionally:

A few additional non-standard attributes are listed following the descriptions of the standard attributes.

accept

Valid for the file input type only, the accept attribute defines which file types are selectable in a file upload control. See the file input type.

alt

Valid for the image button only, the alt attribute provides alternative text for the image, displaying the value of the attribute if the image src is missing or otherwise fails to load. See the image input type.

autocapitalize

Controls whether inputted text is automatically capitalized and, if so, in what manner. See the autocapitalize global attribute page for more information.

autocomplete

(Not a Boolean attribute!) The autocomplete attribute takes as its value a space-separated string that describes what, if any, type of autocomplete functionality the input should provide. A typical implementation of autocomplete recalls previous values entered in the same input field, but more complex forms of autocomplete can exist. For instance, a browser could integrate with a device's contacts list to autocomplete email addresses in an email input field. See autocomplete for permitted values.

The autocomplete attribute is valid on hidden, text, search, url, tel, email, date, month, week, time, datetime-local, number, range, color, and password. This attribute has no effect on input types that do not return numeric or text data, being valid for all input types except checkbox, radio, file, or any of the button types.

See the autocomplete attribute for additional information, including information on password security and how autocomplete is slightly different for hidden than for other input types.

autofocus

A Boolean attribute which, if present, indicates that the input should automatically have focus when the page has finished loading (or when the <dialog> containing the element has been displayed).

Note: An element with the autofocus attribute may gain focus before the DOMContentLoaded event is fired.

No more than one element in the document may have the autofocus attribute. If put on more than one element, the first one with the attribute receives focus.

The autofocus attribute cannot be used on inputs of type hidden, since hidden inputs cannot be focused.

Warning: Automatically focusing a form control can confuse visually-impaired people using screen-reading technology and people with cognitive impairments. When autofocus is assigned, screen-readers "teleport" their user to the form control without warning them beforehand.

Use careful consideration for accessibility when applying the autofocus attribute. Automatically focusing on a control can cause the page to scroll on load. The focus can also cause dynamic keyboards to display on some touch devices. While a screen reader will announce the label of the form control receiving focus, the screen reader will not announce anything before the label, and the sighted user on a small device will equally miss the context created by the preceding content.

capture

Introduced in the HTML Media Capture specification and valid for the file input type only, the capture attribute defines which media—microphone, video, or camera—should be used to capture a new file for upload with file upload control in supporting scenarios. See the file input type.

checked

Valid for both radio and checkbox types, checked is a Boolean attribute. If present on a radio type, it indicates that the radio button is the currently selected one in the group of same-named radio buttons. If present on a checkbox type, it indicates that the checkbox is checked by default (when the page loads). It does not indicate whether this checkbox is currently checked: if the checkbox's state is changed, this content attribute does not reflect the change. (Only the HTMLInputElement's checked IDL attribute is updated.)

Note: Unlike other input controls, a checkboxes and radio buttons value are only included in the submitted data if they are currently checked. If they are, the name and the value(s) of the checked controls are submitted.

For example, if a checkbox whose name is fruit has a value of cherry, and the checkbox is checked, the form data submitted will include fruit=cherry. If the checkbox isn't active, it isn't listed in the form data at all. The default value for checkboxes and radio buttons is on.

dirname

Valid for hidden, text, search, url, tel, and email input types, the dirname attribute enables the submission of the directionality of the element. When included, the form control will submit with two name/value pairs: the first being the name and value, and the second being the value of the dirname attribute as the name, with a value of ltr or rtl as set by the browser.

html

<form action="page.html" method="post">
  <label>
    Fruit:
    <input type="text" name="fruit" dirname="fruit-dir" value="cherry" />
  </label>
  <input type="submit" />
</form>
<!-- page.html?fruit=cherry&fruit-dir=ltr -->

When the form above is submitted, the input cause both the name / value pair of fruit=cherry and the dirname / direction pair of fruit-dir=ltr to be sent. For more information, see the dirname attribute.

disabled

A Boolean attribute which, if present, indicates that the user should not be able to interact with the input. Disabled inputs are typically rendered with a dimmer color or using some other form of indication that the field is not available for use.

Specifically, disabled inputs do not receive the click event, and disabled inputs are not submitted with the form.

Note: Although not required by the specification, Firefox will by default persist the dynamic disabled state of an <input> across page loads. Use the autocomplete attribute to control this feature.

form

A string specifying the <form> element with which the input is associated (that is, its form owner). This string's value, if present, must match the id of a <form> element in the same document. If this attribute isn't specified, the <input> element is associated with the nearest containing form, if any.

The form attribute lets you place an input anywhere in the document but have it included with a form elsewhere in the document.

Note: An input can only be associated with one form.

formaction

Valid for the image and submit input types only. See the submit input type for more information.

formenctype

Valid for the image and submit input types only. See the submit input type for more information.

formmethod

Valid for the image and submit input types only. See the submit input type for more information.

formnovalidate

Valid for the image and submit input types only. See the submit input type for more information.

formtarget

Valid for the image and submit input types only. See the submit input type for more information.

height

Valid for the image input button only, the height is the height of the image file to display to represent the graphical submit button. See the image input type.

id

Global attribute valid for all elements, including all the input types, it defines a unique identifier (ID) which must be unique in the whole document. Its purpose is to identify the element when linking. The value is used as the value of the <label>'s for attribute to link the label with the form control. See <label>.

inputmode

Global value valid for all elements, it provides a hint to browsers as to the type of virtual keyboard configuration to use when editing this element or its contents. Values include none, text, tel, url, email, numeric, decimal, and search.

list

The value given to the list attribute should be the id of a <datalist> element located in the same document. The <datalist> provides a list of predefined values to suggest to the user for this input. Any values in the list that are not compatible with the type are not included in the suggested options. The values provided are suggestions, not requirements: users can select from this predefined list or provide a different value.

It is valid on text, search, url, tel, email, date, month, week, time, datetime-local, number, range, and color.

Per the specifications, the list attribute is not supported by the hidden, password, checkbox, radio, file, or any of the button types.

Depending on the browser, the user may see a custom color palette suggested, tic marks along a range, or even an input that opens like a <select> but allows for non-listed values. Check out the browser compatibility table for the other input types.

See the <datalist> element.

max

Valid for date, month, week, time, datetime-local, number, and range, it defines the greatest value in the range of permitted values. If the value entered into the element exceeds this, the element fails constraint validation. If the value of the max attribute isn't a number, then the element has no maximum value.

There is a special case: if the data type is periodic (such as for dates or times), the value of max may be lower than the value of min, which indicates that the range may wrap around; for example, this allows you to specify a time range from 10 PM to 4 AM.

maxlength

Valid for text, search, url, tel, email, and password, it defines the maximum string length (measured in UTF-16 code units) that the user can enter into the field. This must be an integer value of 0 or higher. If no maxlength is specified, or an invalid value is specified, the field has no maximum length. This value must also be greater than or equal to the value of minlength.

The input will fail constraint validation if the length of the text entered into the field is greater than maxlength UTF-16 code units long. By default, browsers prevent users from entering more characters than allowed by the maxlength attribute. Constraint validation is only applied when the value is changed by the user. See Client-side validation for more information.

min

Valid for date, month, week, time, datetime-local, number, and range, it defines the most negative value in the range of permitted values. If the value entered into the element is less than this, the element fails constraint validation. If the value of the min attribute isn't a number, then the element has no minimum value.

This value must be less than or equal to the value of the max attribute. If the min attribute is present but is not specified or is invalid, no min value is applied. If the min attribute is valid and a non-empty value is less than the minimum allowed by the min attribute, constraint validation will prevent form submission. See Client-side validation for more information.

There is a special case: if the data type is periodic (such as for dates or times), the value of max may be lower than the value of min, which indicates that the range may wrap around; for example, this allows you to specify a time range from 10 PM to 4 AM.

minlength

Valid for text, search, url, tel, email, and password, it defines the minimum string length (measured in UTF-16 code units) that the user can enter into the entry field. This must be a non-negative integer value smaller than or equal to the value specified by maxlength. If no minlength is specified, or an invalid value is specified, the input has no minimum length.

The input will fail constraint validation if the length of the text entered into the field is fewer than minlength UTF-16 code units long, preventing form submission. Constraint validation is only applied when the value is changed by the user. See Client-side validation for more information.

multiple

The Boolean multiple attribute, if set, means the user can enter comma separated email addresses in the email widget or can choose more than one file with the file input. See the email and file input type.

name

A string specifying a name for the input control. This name is submitted along with the control's value when the form data is submitted.

Consider the name a required attribute (even though it's not). If an input has no name specified, or name is empty, the input's value is not submitted with the form! (Disabled controls, unchecked radio buttons, unchecked checkboxes, and reset buttons are also not sent.)

There are two special cases:

1. _charset_ : If used as the name of an <input> element of type hidden, the input's value is automatically set by the user agent to the character encoding being used to submit the form.
2. isindex: For historical reasons, the name isindex is not allowed.

The name attribute creates a unique behavior for radio buttons.

Only one radio button in a same-named group of radio buttons can be checked at a time. Selecting any radio button in that group automatically deselects any currently-selected radio button in the same group. The value of that one checked radio button is sent along with the name if the form is submitted,

When tabbing into a series of same-named group of radio buttons, if one is checked, that one will receive focus. If they aren't grouped together in source order, if one of the group is checked, tabbing into the group starts when the first one in the group is encountered, skipping all those that aren't checked. In other words, if one is checked, tabbing skips the unchecked radio buttons in the group. If none are checked, the radio button group receives focus when the first button in the same name group is reached.

Once one of the radio buttons in a group has focus, using the arrow keys will navigate through all the radio buttons of the same name, even if the radio buttons are not grouped together in the source order.

When an input element is given a name, that name becomes a property of the owning form element's HTMLFormElement.elements property. If you have an input whose name is set to guest and another whose name is hat-size, the following code can be used:

js

let form = document.querySelector("form");

let guestName = form.elements.guest;
let hatSize = form.elements["hat-size"];

When this code has run, guestName will be the HTMLInputElement for the guest field, and hatSize the object for the hat-size field.

Warning: Avoid giving form elements a name that corresponds to a built-in property of the form, since you would then override the predefined property or method with this reference to the corresponding input.

pattern

Valid for text, search, url, tel, email, and password, the pattern attribute is used to compile a regular expression that the input's value must match in order for the value to pass constraint validation. It must be a valid JavaScript regular expression, as used by the RegExp type, and as documented in our guide on regular expressions. No forward slashes should be specified around the pattern text. When compiling the regular expression:

1. the pattern will be implicitly wrapped with ^(?: and )$, such that the match is required against the entire input value, i.e., ^(?:<pattern>)$.
2. the 'v' flag is specified so that the pattern is treated as a sequence of Unicode code points, instead of as ASCII.

If the pattern attribute is present but is not specified or is invalid, no regular expression is applied and this attribute is ignored completely. If the pattern attribute is valid and a non-empty value does not match the pattern, constraint validation will prevent form submission. If the multiple is present, the compiled regular expression is matched against each comma separated value.

Note: If using the pattern attribute, inform the user about the expected format by including explanatory text nearby. You can also include a title attribute to explain what the requirements are to match the pattern; most browsers will display this title as a tooltip. The visible explanation is required for accessibility. The tooltip is an enhancement.

See Client-side validation for more information.

placeholder

Valid for text, search, url, tel, email, password, and number, the placeholder attribute provides a brief hint to the user as to what kind of information is expected in the field. It should be a word or short phrase that provides a hint as to the expected type of data, rather than an explanation or prompt. The text must not include carriage returns or line feeds. So for example if a field is expected to capture a user's first name, and its label is "First Name", a suitable placeholder might be "e.g. Mustafa".

Note: The placeholder attribute is not as semantically useful as other ways to explain your form, and can cause unexpected technical issues with your content. See Labels for more information.

popovertarget

Turns an <input type="button"> element into a popover control button; takes the ID of the popover element to control as its value. See the Popover API landing page for more details.

popovertargetaction

Specifies the action to be performed on a popover element being controlled by a control <input type="button">. Possible values are:

"hide"

The button will hide a shown popover. If you try to hide an already hidden popover, no action will be taken.

"show"

The button will show a hidden popover. If you try to show an already showing popover, no action will be taken.

"toggle"

The button will toggle a popover between showing and hidden. If the popover is hidden, it will be shown; if the popover is showing, it will be hidden. If popovertargetaction is omitted, "toggle" is the default action that will be performed by the control button.

readonly

A Boolean attribute which, if present, indicates that the user should not be able to edit the value of the input. The readonly attribute is supported by the text, search, url, tel, email, date, month, week, time, datetime-local, number, and password input types.

See the HTML attribute: readonly for more information.

required

required is a Boolean attribute which, if present, indicates that the user must specify a value for the input before the owning form can be submitted. The required attribute is supported by text, search, url, tel, email, date, month, week, time, datetime-local, number, password, checkbox, radio, and file inputs.

See Client-side validation and the HTML attribute: required for more information.

size

Valid for email, password, tel, url, and text, the size attribute specifies how much of the input is shown. Basically creates same result as setting CSS width property with a few specialties. The actual unit of the value depends on the input type. For password and text, it is a number of characters (or em units) with a default value of 20, and for others, it is pixels (or px units). CSS width takes precedence over the size attribute.

src

Valid for the image input button only, the src is string specifying the URL of the image file to display to represent the graphical submit button. See the image input type.

step

Valid for date, month, week, time, datetime-local, number, and range, the step attribute is a number that specifies the granularity that the value must adhere to.

If not explicitly included:

• step defaults to 1 for number and range.
• Each date/time input type has a default step value appropriate for the type; see the individual input pages: date, datetime-local, month, time, and week.

The value must be a positive number—integer or float—or the special value any, which means no stepping is implied, and any value is allowed (barring other constraints, such as min and max).

If any is not explicitly set, valid values for the number, date/time input types, and range input types are equal to the basis for stepping — the min value and increments of the step value, up to the max value, if specified.

For example, if you have <input type="number" min="10" step="2">, then any even integer, 10 or greater, is valid. If omitted, <input type="number">, any integer is valid, but floats (like 4.2) are not valid, because step defaults to 1. For 4.2 to be valid, step would have had to be set to any, 0.1, 0.2, or any the min value would have had to be a number ending in .2, such as <input type="number" min="-5.2">

Note: When the data entered by the user doesn't adhere to the stepping configuration, the value is considered invalid in constraint validation and will match the :invalid pseudoclass.

See Client-side validation for more information.

tabindex

Global attribute valid for all elements, including all the input types, an integer attribute indicating if the element can take input focus (is focusable), if it should participate to sequential keyboard navigation. As all input types except for input of type hidden are focusable, this attribute should not be used on form controls, because doing so would require the management of the focus order for all elements within the document with the risk of harming usability and accessibility if done incorrectly.

title

Global attribute valid for all elements, including all input types, containing a text representing advisory information related to the element it belongs to. Such information can typically, but not necessarily, be presented to the user as a tooltip. The title should NOT be used as the primary explanation of the purpose of the form control. Instead, use the <label> element with a for attribute set to the form control's id attribute. See Labels below.

type

A string specifying the type of control to render. For example, to create a checkbox, a value of checkbox is used. If omitted (or an unknown value is specified), the input type text is used, creating a plaintext input field.

Permitted values are listed in Input types above.

value

The input control's value. When specified in the HTML, this is the initial value, and from then on it can be altered or retrieved at any time using JavaScript to access the respective HTMLInputElement object's value property. The value attribute is always optional, though should be considered mandatory for checkbox, radio, and hidden.

width

Valid for the image input button only, the width is the width of the image file to display to represent the graphical submit button. See the image input type.

The following non-standard attributes are also available on some browsers. As a general rule, you should avoid using them unless it can't be helped.

incremental Non-standard

The Boolean attribute incremental is a WebKit and Blink extension (so supported by Safari, Opera, Chrome, etc.) which, if present, tells the user agent to process the input as a live search. As the user edits the value of the field, the user agent sends search events to the HTMLInputElement object representing the search box. This allows your code to update the search results in real time as the user edits the search.

If incremental is not specified, the search event is only sent when the user explicitly initiates a search (such as by pressing the Enter or Return key while editing the field).

The search event is rate-limited so that it is not sent more frequently than an implementation-defined interval.

orient Non-standard

Similar to the -moz-orient non-standard CSS property impacting the <progress> and <meter> elements, the orient attribute defines the orientation of the range slider. Values include horizontal, meaning the range is rendered horizontally, and vertical, where the range is rendered vertically. See Creating vertical form controls for a modern approach to creating vertical form controls.

results Non-standard

The results attribute—supported only by Safari—is a numeric value that lets you override the maximum number of entries to be displayed in the <input> element's natively-provided drop-down menu of previous search queries.

The value must be a non-negative decimal number. If not provided, or an invalid value is given, the browser's default maximum number of entries is used.

webkitdirectory Non-standard

The Boolean webkitdirectory attribute, if present, indicates that only directories should be available to be selected by the user in the file picker interface. See HTMLInputElement.webkitdirectory for additional details and examples.

Though originally implemented only for WebKit-based browsers, webkitdirectory is also usable in Microsoft Edge as well as Firefox 50 and later. However, even though it has relatively broad support, it is still not standard and should not be used unless you have no alternative.

The following methods are provided by the HTMLInputElement interface which represents <input> elements in the DOM. Also available are those methods specified by the parent interfaces, HTMLElement, Element, Node, and EventTarget.

checkValidity()

Returns true if the element's value passes validity checks; otherwise, returns false and fires an invalid event at the element.

reportValidity()

Returns true if the element's value passes validity checks; otherwise, returns false, fires an invalid event at the element, and (if the event isn't canceled) reports the problem to the user.

select()

Selects the entire content of the <input> element, if the element's content is selectable. For elements with no selectable text content (such as a visual color picker or calendar date input), this method does nothing.

setCustomValidity()

Sets a custom message to display if the input element's value isn't valid.

setRangeText()

Sets the contents of the specified range of characters in the input element to a given string. A selectMode parameter is available to allow controlling how the existing content is affected.

setSelectionRange()

Selects the specified range of characters within a textual input element. Does nothing for inputs which aren't presented as text input fields.

showPicker()

Displays the browser picker for the input element that would normally be displayed when the element is selected, but triggered from a button press or other user interaction.

stepDown()

Decrements the value of a numeric input by one, by default, or by the specified number of units.

stepUp()

Increments the value of a numeric input by one or by the specified number of units.

Inputs, being replaced elements, have a few features not applicable to non form elements. There are CSS selectors that can specifically target form controls based on their UI features, also known as UI pseudo-classes. The input element can also be targeted by type with attribute selectors. There are some properties that are especially useful as well.

Pseudo-classes example

We can style a checkbox label based on whether the checkbox is checked or not. In this example, we are styling the color and font-weight of the <label> that comes immediately after a checked input. We haven't applied any styles if the input is not checked.

<input id="checkboxInput" type="checkbox" />
<label for="checkboxInput">Toggle the checkbox on and off</label>

input:checked + label {
  color: red;
  font-weight: bold;
}

It is possible to target different types of form controls based on their type using attribute selectors. CSS attribute selectors match elements based on either just the presence of an attribute or the value of a given attribute.

/* matches a password input */
input[type="password"] {
}

/* matches a form control whose valid values are limited to a range of values*/
input[min][max] {
}

/* matches a form control with a pattern attribute */
input[pattern] {
}

By default, the appearance of placeholder text is a translucent or light gray. The ::placeholder pseudo-element is the input's placeholder text. It can be styled with a limited subset of CSS properties.

::placeholder {
  color: blue;
}

Only the subset of CSS properties that apply to the ::first-line pseudo-element can be used in a rule using ::placeholder in its selector.

The appearance property enables the displaying of (almost) any element as a platform-native style based on the operating system's theme as well as the removal of any platform-native styling with the none value.

You could make a <div> look like a radio button with div {appearance: radio;} or a radio look like a checkbox with [type="radio"] {appearance: checkbox;}, but don't.

Setting appearance: none removes platform native borders, but not functionality.

A property specific to text entry-related elements is the CSS caret-color property, which lets you set the color used to draw the text input caret:

HTML

<label for="textInput">Note the red caret:</label>
<input id="textInput" class="custom" size="32" />

CSS

input.custom {
  caret-color: red;
  font:
    16px "Helvetica",
    "Arial",
    "sans-serif";
}

Result

The field-sizing property enables you to control the sizing behavior of form inputs (i.e. they are given a default preferred size by default.) This property enables you to override the default behavior, allowing form controls to adjust in size to fit their contents.

This property is typically used to create form fields that shrinkwrap their content and grow as more text is entered. This works with input types that accept direct text input (for example, text and url), input type file, and <textarea> elements.

In certain cases (typically involving non-textual inputs and specialized interfaces), the <input> element is a replaced element. When it is, the position and size of the element's size and positioning within its frame can be adjusted using the CSS object-position and object-fit properties.

For more information about adding color to elements in HTML, see:

• Applying color to HTML elements using CSS.

Also see:

• Styling HTML forms
• Advanced styling for HTML forms and
• the compatibility table of CSS properties.

Labels are needed to associate assistive text with an <input>. The <label> element provides explanatory information about a form field that is always appropriate (aside from any layout concerns you have). It's never a bad idea to use a <label> to explain what should be entered into an <input> or <textarea>.

Associated labels

The semantic pairing of <input> and <label> elements is useful for assistive technologies such as screen readers. By pairing them using the <label>'s for attribute, you bond the label to the input in a way that lets screen readers describe inputs to users more precisely.

It does not suffice to have plain text adjacent to the <input> element. Rather, usability and accessibility requires the inclusion of either implicit or explicit <label>:

<!-- inaccessible -->
<p>Enter your name: <input id="name" type="text" size="30" /></p>

<!-- implicit label -->
<p>
  <label>Enter your name: <input id="name" type="text" size="30" /></label>
</p>

<!-- explicit label -->
<p>
  <label for="name">Enter your name: </label>
  <input id="name" type="text" size="30" />
</p>

The first example is inaccessible: no relationship exists between the prompt and the <input> element.

In addition to an accessible name, the label provides a larger 'hit' area for mouse and touch screen users to click on or touch. By pairing a <label> with an <input>, clicking on either one will focus the <input>. If you use plain text to "label" your input, this won't happen. Having the prompt part of the activation area for the input is helpful for people with motor control conditions.

As web developers, it's important that we never assume that people will know all the things that we know. The diversity of people using the web—and by extension your website—practically guarantees that some of your site's visitors will have some variation in thought processes and/or circumstances that leads them to interpret your forms very differently from you without clear and properly-presented labels.

Placeholders are not accessible

The placeholder attribute lets you specify text that appears within the <input> element's content area itself when it is empty. The placeholder should never be required to understand your forms. It is not a label, and should not be used as a substitute, because it isn't. The placeholder is used to provide a hint as to what an inputted value should look like, not an explanation or prompt.

Not only is the placeholder not accessible to screen readers, but once the user enters any text into the form control, or if the form control already has a value, the placeholder disappears. Browsers with automatic page translation features may skip over attributes when translating, meaning the placeholder may not get translated.

In addition to using CSS to style inputs based on the :valid or :invalid UI states based on the current state of each input, as noted in the UI pseudo-classes section above, the browser provides for client-side validation on (attempted) form submission. On form submission, if there is a form control that fails constraint validation, supporting browsers will display an error message on the first invalid form control; displaying a default message based on the error type, or a message set by you.

Some input types and other attributes place limits on what values are valid for a given input. For example, <input type="number" min="2" max="10" step="2"> means only the number 2, 4, 6, 8, or 10 are valid. Several errors could occur, including a rangeUnderflow error if the value is less than 2, rangeOverflow if greater than 10, stepMismatch if the value is a number between 2 and 10, but not an even integer (does not match the requirements of the step attribute), or typeMismatch if the value is not a number.

For the input types whose domain of possible values is periodic (that is, at the highest possible value, the values wrap back around to the beginning rather than ending), it's possible for the values of the max and min properties to be reversed, which indicates that the range of permitted values starts at min, wraps around to the lowest possible value, then continues on until max is reached. This is particularly useful for dates and times, such as when you want to allow the range to be from 8 PM to 8 AM:

<input type="time" min="20:00" max="08:00" name="overnight" />

Specific attributes and their values can lead to a specific error ValidityState:

If a form control doesn't have the required attribute, no value, or an empty string, is not invalid. Even if the above attributes are present, with the exception of required, an empty string will not lead to an error.

We can set limits on what values we accept, and supporting browsers will natively validate these form values and alert the user if there is a mistake when the form is submitted.

In addition to the errors described in the table above, the validityState interface contains the badInput, valid, and customError boolean readonly properties. The validity object includes:

• validityState.valueMissing
• validityState.typeMismatch
• validityState.patternMismatch
• validityState.tooLong
• validityState.tooShort
• validityState.rangeUnderflow
• validityState.rangeOverflow
• validityState.stepMismatch
• validityState.badInput
• validityState.valid
• validityState.customError

For each of these Boolean properties, a value of true indicates that the specified reason validation may have failed is true, with the exception of the valid property, which is true if the element's value obeys all constraints.

If there is an error, supporting browsers will both alert the user and prevent the form from being submitted. A word of caution: if a custom error is set to a truthy value (anything other than the empty string or null), the form will be prevented from being submitted. If there is no custom error message, and none of the other properties return true, valid will be true, and the form can be submitted.

function validate(input) {
  let validityState_object = input.validity;
  if (validityState_object.valueMissing) {
    input.setCustomValidity("A value is required");
  } else if (validityState_object.rangeUnderflow) {
    input.setCustomValidity("Your value is too low");
  } else if (validityState_object.rangeOverflow) {
    input.setCustomValidity("Your value is too high");
  } else {
    input.setCustomValidity("");
  }
}

The last line, setting the custom validity message to the empty string is vital. If the user makes an error, and the validity is set, it will fail to submit, even if all the values are valid, until the message is null.

Custom validation error example

If you want to present a custom error message when a field fails to validate, you need to use the Constraint Validation API available on <input> (and related) elements. Take the following form:

<form>
  <label for="name">Enter username (upper and lowercase letters): </label>
  <input type="text" name="name" id="name" required pattern="[A-Za-z]+" />
  <button>Submit</button>
</form>

The basic HTML form validation features will cause this to produce a default error message if you try to submit the form with either no valid filled in, or a value that does not match the pattern.

If you wanted to instead display custom error messages, you could use JavaScript like the following:

const nameInput = document.querySelector("input");

nameInput.addEventListener("input", () => {
  nameInput.setCustomValidity("");
  nameInput.checkValidity();
});

nameInput.addEventListener("invalid", () => {
  if (nameInput.value === "") {
    nameInput.setCustomValidity("Enter your username!");
  } else {
    nameInput.setCustomValidity(
      "Usernames can only contain upper and lowercase letters. Try again!",
    );
  }
});

The example renders like so:

In brief:

• We check the valid state of the input element every time its value is changed by running the checkValidity() method via the input event handler.
• If the value is invalid, an invalid event is raised, and the invalid event handler function is run. Inside this function we work out whether the value is invalid because it is empty, or because it doesn't match the pattern, using an if () block, and set a custom validity error message.
• As a result, if the input value is invalid when the submit button is pressed, one of the custom error messages will be shown.
• If it is valid, it will submit as you'd expect. For this to happen, the custom validity has to be cancelled, by invoking setCustomValidity() with an empty string value. We therefore do this every time the input event is raised. If you don't do this, and a custom validity was previously set, the input will register as invalid, even if it currently contains a valid value on submission.

The allowed inputs for certain <input> types depend on the locale. In some locales, 1,000.00 is a valid number, while in other locales the valid way to enter this number is 1.000,00.

Firefox uses the following heuristics to determine the locale to validate the user's input (at least for type="number"):

• Try the language specified by a lang/xml:lang attribute on the element or any of its parents.
• Try the language specified by any Content-Language HTTP header. Or,
• If none specified, use the browser's locale.

When including inputs, it is an accessibility requirement to add labels alongside. This is needed so those who use assistive technologies can tell what the input is for. Also, clicking or touching a label gives focus to the label's associated form control. This improves the accessibility and usability for sighted users, increases the area a user can click or touch to activate the form control. This is especially useful (and even needed) for radio buttons and checkboxes, which are tiny. For more information about labels in general see Labels.

The following is an example of how to associate the <label> with an <input> element in the above style. You need to give the <input> an id attribute. The <label> then needs a for attribute whose value is the same as the input's id.

<label for="peas">Do you like peas?</label>
<input type="checkbox" name="peas" id="peas" />

Interactive elements such as form input should provide an area large enough that it is easy to activate them. This helps a variety of people, including people with motor control issues and people using non-precise forms of input such as a stylus or fingers. A minimum interactive size of 44×44 CSS pixels is recommended.

• Understanding Success Criterion 2.5.5: Target Size | W3C Understanding WCAG 2.1
• Target Size and 2.5.5 | Adrian Roselli
• Quick test: Large touch targets - The A11Y Project

• Form constraint validation
• Your first HTML form
• How to structure an HTML form
• The native form widgets
• Sending form data
• Form data validation
• How to build custom form widgets
• HTML forms in legacy browsers
• Styling HTML forms
• Advanced styling for HTML forms
• CSS property compatibility table
• Creating vertical form controls