What’s in the head? Metadata in HTML

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O head de um documento HTML é a parte que não é mostrada no navegador web quando a página é carregada.Ele contém informações como o título da página, links para o CSS (se você quiser estilizar seu conteúdo HTML com CSS), links para ícones personalizados, e outros metadados (informações sobre o documento HTML, como quem o escreveu, e importantes palavras-chave que descrevem o documento.) Nesse artigo vamos falar sobre todas as coisas acima e mais, para lhe dar uma boa base para lidar com marcação e outros códigos que podem vir à sua mente.

Prerequisites: Basic HTML familiarity, as covered in Getting started with HTML.
Objective: To learn about the HTML head, what its purpose is, the most important items it can contain, and what effect it can have on the HTML document.

O que há no cabeçalho HTML?

Vamos voltar no simples documento HTML que nós vimos no artigo anterior:

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>My test page</title>
  </head>
  <body>
    <p>This is my page</p>
  </body>
</html>

O cabeçalho HTML é o conteúdo do elemento <head>  — ao contrário dos conteúdos do elemento <body>  (que são exibidos na página quando carregados no navegador), o conteúdo do cabeçalho não é exibido na página. Em vez disso, o trabalho do cabeçalho é conter metadata sobre o documento. No exemplo sequinte, o cabeçalho é bem simples:

<head>
  <meta charset="utf-8">
  <title>My test page</title>
</head>

Em páginas maiores, o cabeçalho pode ter mais conteúdo — tente acessar um dos seus sites favoritos e use as ferramentas de desenvolvedor para verificar o conteúdo do cabeçalho. Nosso objetivo aqui não é mostrar a você como usar tudo o que é possível pôr no cabeçalho, mas te ensinar a usar as coisas mais evidentes que você vai querer incluir no cabeçalho, e lhe dar alguma familiaridade. Vamos começar.

Adicionando um título

Nós já vimos o elemento <title> em ação — ele pode ser usado para adicionar um título ao documento. Isso, entretanto, pode ser confundido com o elemento <h1> , que é usado para adicionar um título de nível superior ao conteúdo do body — as vezes também é associado como o título da página. Mas são coisas diferentes!

  • O elemento <h1>  aparece na página quando é carregado no navegador — geralmente isso deve ser usado uma vez por página, para marcar o título do conteúdo da sua página (o título da história, ou da notícia, ou o que quer que seja apropriado para o uso.)
  • O elemento <title> é um metadado que representa o título de todo o document HTML (não o conteúdo do documento.)

Aprendizado ativo: observando um exemplo simples

  1. Para iniciar esse aprendizado, gostaríamos que você fosse ao nosso repositório do Github e baixar uma cópia do nosso title-example.html page. Para fazer isso,
    1. Copie e cole o código em um novo arquivo de texto no seu editor de código e salve-o em um lugar de fácil acesso.
    2. Aperto o botão "Raw na página", vá até Arquivo > Salvar Página como... no menu di seu navegador, então escolha um lugar para salvar seu arquivo.
  2. Agora abra o arquivo no seu navegador. Você deve ver algo assim:

    A simple web page with the title set to <title> element, and the <h1> set to <h1> element.Pode parecer óbivio agora onde o conteúdo do <h1> aparece, e onde o conteúdo do  <title> aparece!

  3. Você deveria também tentar abrir o código no seu editor de código, editar o conteúdo desses elementos, e depois atualizar a página no navegador. Divirta-se.

Os conteúdos do elemento <title> são usados também de outra maneira. Por exemplo, se você tentar adicionar uma página aos favoritos (Favoritos > adicionar aos favoritos, no Firefox), você verá o conteúdo do <title>  preenchido como favorito sugerido.

A webpage being bookmarked in firefox; the bookmark name has been automatically filled in with the contents of the <title> element

The <title> contents are also used in search results, as you'll see below.

Metadata: the <meta> element

Metadata is data that describes data, and HTML has an "official" way of adding metadata to a document — the <meta> element. Of course, the other stuff we are talking about in this article could also be thought of as metadata too. There are a lot of different types of <meta> element that can be included in your page's <head>, but we won't try to explain them all at this stage, as it would just get too confusing. Instead, we'll explain a few things that you might commonly see, just to give you an idea.

Specifying your document's character encoding

In the example we saw above, this line was included:

<meta charset="utf-8">

This element simply specifies the document's character encoding — the character set that the document is permitted to use. utf-8 is a universal character set that includes pretty much any character from any human language. This means that your web page will be able to handle displaying any language; it's therefore a good idea to set this on every web page you create! For example, your page could handle English and Japanese just fine:

a web page containing English and Japanese characters, with the character encoding set to universal, or utf-8. Both languages display fine,If you set your character encoding to ISO-8859-1, for example (the character set for the Latin alphabet), your page rendering would be all messed up:

a web page containing English and Japanese characters, with the character encoding set to latin. The Japanese characters don't display correctly

Active learning: Experiment with character encoding

To try this out, revisit the simple HTML template you obtained in the previous section on <title> (the title-example.html page), try changing the meta charset value to ISO-8859-1, and add the Japanese to your page. This is the code we used:

<p>Japanese example: ご飯が熱い。</p>

Adding an author and description

Many <meta> elements include name and content attributes:

  • name specifies the type of meta element it is; what type of information it contains.
  • content specifies the actual meta content.

Two such meta elements that are useful to include on your page define the author of the page, and provide a concise description of the page. Let's look at an example:

<meta name="author" content="Chris Mills">
<meta name="description" content="The MDN Learning Area aims to provide
complete beginners to the Web with all they need to know to get
started with developing web sites and applications.">

Specifying an author is useful in a few ways: it is useful to be able to work out who wrote the page, if you want to contact them with questions about the content. Some content management systems have facilities to automatically extract page author information and make it available for such purposes.

Specifying a description that includes keywords relating to the content of your page is useful as it has the potential to make your page appear higher in relevant searches performed in search engines (such activities are termed Search Engine Optimization, or SEO.)

Active learning: The description's use in search engines

The description is also used on search engine result pages. Let's go through an exercise to explore this

  1. Go to the front page of The Mozilla Developer Network.
  2. View the page's source (Right/Ctrl + click on the page, choose View Page Source from the context menu.)
  3. Find the description meta tag. It will look like this:
    <meta name="description" content="The Mozilla Developer Network (MDN) provides
    information about Open Web technologies including HTML, CSS, and APIs for both
    Web sites and HTML5 Apps. It also documents Mozilla products, like Firefox OS.">
  4. Now search for "Mozilla Developer Network" in your favourite search engine (We used Yahoo.) You'll notice the description <meta> and <title> element content used in the search result — definitely worth having!

    A Yahoo search result for "Mozilla Developer Network"

Note: In Google, you will see some relevant subpages of MDN listed below the main MDN homepage link — these are called sitelinks, and are configurable in Google's webmaster tools — a way to make your site's search results better in the Google search engine.

Note: Many <meta> features just aren't used any more. For example, the keyword <meta> element — which is supposed to provide keywords for search engines to determine relevance of that page for different search terms — is ignored by search engines, because spammers were just filling the keyword list with hundreds of keywords, biasing results.

Other types of metadata

As you trawl around the Web, you'll find other types of metadata, too. A lot of the features you'll see on websites are proprietary creations, designed to provide certain sites (such as social networking sites) with specific pieces of information they can use.

For example, Open Graph Data is a metadata protocol that Facebook invented to provide richer metadata for websites. In the MDN sourcecode, you'll find this:

<meta property="og:image" content="https://developer.cdn.mozilla.net/static/img/opengraph-logo.dc4e08e2f6af.png">
<meta property="og:description" content="The Mozilla Developer Network (MDN) provides
information about Open Web technologies including HTML, CSS, and APIs for both Web sites
and HTML5 Apps. It also documents Mozilla products, like Firefox OS.">
<meta property="og:title" content="Mozilla Developer Network">

One effect of this is that when you link to MDN on facebook, the link appears along with an image and description: a richer experience for users.

Open graph protocol data from the MDN homepage as displayed on facebook, showing an image, title, and description.Twitter also has its own similar proprietary metadata, which has a similar effect when the site's URL is displayed on twitter.com. For example:

<meta name="twitter:title" content="Mozilla Developer Network">

Adding custom icons to your site

To further enrich your site design, you can add references to custom icons in your metadata, and these will be displayed in certain contexts.

The humble favicon, which has been around for many, many years, was the first icon of this type, a 16 x 16 pixel icon used in multiple places. The favicon can be added to your page by:

  1. Saving it in the same directory as the site's index page, saved in .ico format (most browsers will support favicons in more common formats like .gif or .png, but using the ICO format will ensure it works as far back as Internet Explorer 6.)
  2. Adding the following line into your HTML <head> to reference it:
    <link rel="shortcut icon" href="favicon.ico" type="image/x-icon">

Modern browsers use favicons in various places, such as in the tab the page is open in, and in the bookmarks panel when you bookmark the page:

The Firefox bookmarks panel, showing a bookmarked example with a favicon displayed next to it.

There are lots of other icon types to consider these days as well. For example, you'll find this in the source code of the MDN homepage:

<!-- third-generation iPad with high-resolution Retina display: -->
<link rel="apple-touch-icon-precomposed" sizes="144x144" href="https://developer.cdn.mozilla.net/static/img/favicon144.a6e4162070f4.png">
<!-- iPhone with high-resolution Retina display: -->
<link rel="apple-touch-icon-precomposed" sizes="114x114" href="https://developer.cdn.mozilla.net/static/img/favicon114.0e9fabd44f85.png">
<!-- first- and second-generation iPad: -->
<link rel="apple-touch-icon-precomposed" sizes="72x72" href="https://developer.cdn.mozilla.net/static/img/favicon72.8ff9d87c82a0.png">
<!-- non-Retina iPhone, iPod Touch, and Android 2.1+ devices: -->
<link rel="apple-touch-icon-precomposed" href="https://developer.cdn.mozilla.net/static/img/favicon57.a2490b9a2d76.png">
<!-- basic favicon -->
<link rel="shortcut icon" href="https://developer.cdn.mozilla.net/static/img/favicon32.e02854fdcf73.png">

The comments explain what each icon is used for — these elements cover things like providing a nice high resolution icon to use when the website is saved to an iPad's home screen.

Don't worry too much about implementing all these types of icon right now — this is a fairly advanced feature, and you won't be expected to have knowledge of this to progress through the course. The main purpose here is to let you know what such things are, in case you come across them while browsing other web sites' source code.

Applying CSS and JavaScript to HTML

Just about all websites you'll use in the modern day will employ CSS to make them look cool, and JavaScript to power interactive functionality, such as video players, maps, games, and more. These are most commonly applied to a web page using the <link> element and the <script> element, respectively.

  • The <link> element always goes inside the head of your document. This takes two attributes, rel="stylesheet", which indicates that it is the document's stylesheet, and href, which contains the path to the stylesheet file:

    <link rel="stylesheet" href="my-css-file.css">
  • The <script> element does not have to go in the head; in fact, often it is better to put it at the bottom of the document body (just before the closing </body> tag), to make sure that all the HTML content has been read by the browser before it tries to apply JavaScript to it (if JavaScript tries to access an element that doesn't yet exist, the browser will throw an error.)

    <script src="my-js-file.js"></script>

    Note: The <script> element may look like an empty element, but it's not, and so needs a closing tag. Instead of pointing to an external script file, you can also choose to put your script inside the <script> element.

Active learning: applying CSS and JavaScript to a page

  1. To start this active learning, grab a copy of our meta-example.html, script.js and style.css files, and save them on your local computer in the same directory. Make sure they are saved with the correct names and file extensions.
  2. Open the HTML file in both your browser, and your text editor.
  3. By following the information given above, add <link> and <script> elements to your HTML, so that your CSS and JavaScript are applied to your HTML.

If done correctly, when you save your HTML and refresh your browser you'll see that things have changed:

Example showing a page with CSS and JavaScript applied to it. The CSS has made the page go green, whereas the JavaScript has added a dynamic list to the page.

  • The JavaScript has added an empty list to the page. Now when you click anywhere on the list, a dialog box will pop up asking you to enter some text for a new list item. when you press the OK button, a new list item will be added to the list containing the text. When you click on an existing list item, a dialog box will pop up allowing you to change the item's text.
  • The CSS has caused the background to go green, and the text to become bigger. It has also styled some of the content that the JavaScript has added to the page (the red bar with the black border is the styling the CSS has added to the JS-generated list.)

Note: If you get stuck in this exercise and can't get the CSS/JS to apply, try checking out our css-and-js.html example page.

Setting the primary language of the document

Finally, it's worth mentioning that you can (and really should) set the language of your page. This can be done by adding the lang attribute to the opening HTML tag (as seen in the meta-example.html and shown below.)

<html lang="en-US">

This is useful in many ways. Your HTML document will be indexed more effectively by search engines if its language is set (allowing it to appear correctly in language-specific results, for example), and it is useful to people with visual impairments using screen readers (for example, the word "six" exists in both French and English, but is pronounced differently.)

You can also set sub sections of your document to be recognised as different languages. For example, we could set our Japanese language section to be recognised as Japanese, like so:

<p>Japanese example: <span lang="jp">ご飯が熱い。</span>.</p>

These codes are defined by the ISO 639-1 standard. You can find more about them in Language tags in HTML and XML.

Summary

That marks the end of our quickfire tour of the HTML head — there's a lot more you can do in here, but an exhaustive tour would be boring and confusing at this stage, and we just wanted to give you an idea of the most common things you'll find in there for now! In the next article we'll be looking at HTML text fundamentals.

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