<a>

HTML <a> 元素(意為 Anchor)建立了通往其他頁面、檔案、Email 地址、或其他 URL 的超連結。

屬性

本元素包含全域屬性 (en-US)

download

Causes the browser to treat the linked URL as a download. Can be used with or without a value:

  • Without a value, the browser will suggest a filename/extension, generated from various sources:
  • Defining a value suggests it as the filename. / and \ characters are converted to underscores (_). Filesystems may forbid other characters in filenames, so browsers will adjust the suggested name if necessary.

Note:

  • download only works for same-origin URLs, or the blob: and data: schemes.
  • How browsers treat downloads varies by browser, user settings, and other factors. The user may be prompted before a download starts, or the file may be saved automatically, or it may open automatically, either in an external application or in the browser itself.
  • If the Content-Disposition header has different information from the download attribute, resulting behavior may differ:
    • If the header specifies a filename, it takes priority over a filename specified in the download attribute.
    • If the header specifies a disposition of inline, Chrome and Firefox prioritize the attribute and treat it as a download. Old Firefox versions (before 82) prioritize the header and will display the content inline.
href

The URL that the hyperlink points to. Links are not restricted to HTTP-based URLs — they can use any URL scheme supported by browsers:

  • Sections of a page with fragment URLs
  • Pieces of media files with media fragments
  • Telephone numbers with tel: URLs
  • Email addresses with mailto: URLs
  • While web browsers may not support other URL schemes, web sites can with registerProtocolHandler()
hreflang

Hints at the human language of the linked URL. No built-in functionality. Allowed values are the same as the global lang attribute (en-US).

ping

A space-separated list of URLs. When the link is followed, the browser will send POST requests with the body PING to the URLs. Typically for tracking.

referrerpolicy

How much of the referrer (en-US) to send when following the link.

  • no-referrer: The Referer (en-US) header will not be sent.
  • no-referrer-when-downgrade: The Referer (en-US) header will not be sent to origin (en-US)s without TLS (en-US) (HTTPS (en-US)).
  • origin: The sent referrer will be limited to the origin of the referring page: its scheme (en-US), host, and port.
  • origin-when-cross-origin: The referrer sent to other origins will be limited to the scheme, the host, and the port. Navigations on the same origin will still include the path.
  • same-origin: A referrer will be sent for same origin (en-US), but cross-origin requests will contain no referrer information.
  • strict-origin: Only send the origin of the document as the referrer when the protocol security level stays the same (HTTPS→HTTPS), but don't send it to a less secure destination (HTTPS→HTTP).
  • strict-origin-when-cross-origin (default): Send a full URL when performing a same-origin request, only send the origin when the protocol security level stays the same (HTTPS→HTTPS), and send no header to a less secure destination (HTTPS→HTTP).
  • unsafe-url: The referrer will include the origin and the path (but not the fragment (en-US), password (en-US), or username (en-US)). This value is unsafe, because it leaks origins and paths from TLS-protected resources to insecure origins.
rel

The relationship of the linked URL as space-separated link types (en-US).

target

Where to display the linked URL, as the name for a browsing context (a tab, window, or <iframe> (en-US)). The following keywords have special meanings for where to load the URL:

  • _self: the current browsing context. (Default)
  • _blank: usually a new tab, but users can configure browsers to open a new window instead.
  • _parent: the parent browsing context of the current one. If no parent, behaves as _self.
  • _top: the topmost browsing context (the "highest" context that's an ancestor of the current one). If no ancestors, behaves as _self.

Note: Setting target="_blank" on <a> elements implicitly provides the same rel behavior as setting rel="noopener" (en-US) which does not set window.opener. See browser compatibility for support status.

type

Hints at the linked URL's format with a MIME type. No built-in functionality.

Obsolete

charset 已棄用

This attribute defined the character encoding of the linked URL. The value should be a space- and/or comma-delimited list of character sets defined in RFC 2045. The default value is ISO-8859-1.

備註: This attribute is obsolete in HTML5 and should not be used by authors. To achieve its effect, use the HTTP Content-Type: header on the linked URL.

coords 已棄用

For use with the below shape attribute, this attribute used a comma-separated list of numbers to define the coordinates of the link on the page.

name 已棄用

This attribute was required for anchors defining a possible target location within a page. In HTML 4.01, id and name could be used simultaneously on a <a> element as long as they have identical values.

備註: This attribute is obsolete in HTML5, use the global attribute id (en-US) instead.

rev 已棄用

This attribute specified a reverse link, the inverse relationship of the rel attribute. It was deprecated for being very confusing.

shape 已棄用

This attribute was used to define a region for hyperlinks to create an image map. The values are circle, default, polygon, and rect. The format of the coords attribute depends on the value of shape. For circle, the value is x,y,r where x and y are the pixel coordinates for the center of the circle and r is the radius value in pixels. For rect, the coords attribute should be x,y,w,h. The x,y values define the upper-left-hand corner of the rectangle, while w and h define the width and height respectively. A value of polygon for shape requires x1,y1,x2,y2,... values for coords. Each of the x,y pairs defines a point in the polygon, with successive points being joined by straight lines and the last point joined to the first. The value default for shape requires that the entire enclosed area, typically an image, be used.

備註: Use the usemap attribute (en-US) for the <img> (en-US) element and the associated <map> (en-US) element to define hotspots instead of the shape attribute.

Examples

Linking to an external location

<!-- anchor linking to external file -->
<a href="https://www.mozilla.com/">
External Link
</a>

Result

External Link

Linking to another section on the same page

<!-- links to element on this page with id="attr-href" -->
<a href="#attr-href">
Description of Same-Page Links
</a>

Result

Description of Same Page Links

Creating a clickable image

This example uses an image to link to the MDN home page. The home page will open in a new browsing context, that is, a new page or a new tab.

<a href="https://developer.mozilla.org/en-US/" target="_blank">
  <img src="https://mdn.mozillademos.org/files/6851/mdn_logo.png"
       alt="MDN logo" />
</a>

Result

It's common to create links that open in the user's email program to allow them to send a new message. This is done with a mailto: link. Here's a simple example:

<a href="mailto:nowhere@mozilla.org">Send email to nowhere</a>

Result

Send email to nowhere

For additional details about the mailto URL scheme, such as including the subject, body, or other predetermined content, see Email links (en-US) or RFC 6068.

Offering phone links is helpful for users viewing web documents and laptops connected to phones.

<a href="tel:+491570156">+49 157 0156</a>

For additional details about the tel URL scheme, see RFC 3966.

Using the download attribute to save a <canvas> as a PNG

If you want to let users download an HTML <canvas> element as an image, you can create a link with a download attribute and the canvas data as a file URL:

var link = document.createElement('a');
link.textContent = 'download image';

link.addEventListener('click', function(ev) {
    link.href = canvas.toDataURL();
    link.download = "mypainting.png";
}, false);

document.body.appendChild(link);

You can see this in action at jsfiddle.net/codepo8/V6ufG/2/.

Notes

HTML 3.2 defines only the name, href, rel, rev, and title attributes.

Accessibility recommendations

Anchor tags are often abused with the onclick event to create pseudo-buttons by setting href to "#" or "javascript:void(0)" to prevent the page from refreshing. These values cause unexpected behavior when copying/dragging links, opening links in a new tabs/windows, bookmarking, and when JavaScript is still downloading, errors out, or is disabled. This also conveys incorrect semantics to assistive technologies (e.g., screen readers). In these cases, it is recommended to use a <button> instead. In general you should only use an anchor for navigation using a proper URL.

Clicking and focus

Whether clicking on an <a> causes it to become focused varies by browser and OS.

Desktop Browsers Windows 8.1 OS X 10.9
Firefox 30.0 Yes Yes
Chrome ≥39 (Chromium bug 388666) Yes Yes
Safari 7.0.5 N/A Only when it has a tabindex
Internet Explorer 11 Yes N/A
Presto (Opera 12) Yes Yes
Mobile Browsers iOS 7.1.2 Android 4.4.4
Safari Mobile Only when it has a tabindex N/A
Chrome 35 ??? Only when it has a tabindex

Specifications

Specification
HTML Standard
# the-a-element

Browser compatibility

BCD tables only load in the browser

See also