C'est quoi le head? Metadonnees en HTML

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Le head d'un document HTML est la partie qui ne sera pas affiché dns le navigateur quand la page est chargée. Il contient des comme le <title> de la page, Le lien du style  CSS (si vous voulez styliser votre contenu HTML avec le CSS), Les liens pour personnaliser les favicons, et autres métadonnées(les informations sur le code, comme l'auteur, les mots clés qui caracterise le document.) Dans cet article, nous allons couvrir toutes les choses ci-dessus et plus, pour vous donner une bonne base pour traiter les balises et d'autres codes qui devraient être dans votre la balise head.

Prérequis: Familier avec le HTML Basic, comme vu dans l'article précedent Commencer avec le HTML
Objectifs: En savoir plus sur la balise head, son objet, les éléments les plus importants qu'elle peut contenir et l'effet qu'elle peut avoir sur le document HTML.

C'est quoi la balise head de HTML?

Revoyons le HTML de base qu'on a vu dans l' Article precedent:

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Ma page test</title>
  </head>
  <body>
    <p>Voici ma page</p>
  </body>
</html>

Le contenu de la balise HTML <head> — à la difference du contenu de la balise <body> (qui est affiché dans le navigateur quand la page est chargée), n'est pas affiché dans la page du navigateur. Le travail de la balise head est de contenir les métadonnée à propos du document. Dans l'exemple en dessus, le head est plutôt petit:

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

 

Dans la majeure partie des pages, le head peut obtenir assez plein d'éléments - essayez d'aller sur certains de vos sites Web préférés et en utilisant les outils de développement pour vérifier leur contenu head. Notre objectif ici n'est pas de vous montrer comment utiliser tout ce qui peut être mis dans la balise head, mais plutôt de vous apprendre à utiliser les choses les plus évidentes que vous voudrez inclure dans le head, et vous donner une certaine familiarité. Commençons.

Ajouter un titre

Nous avons déjà vu l'élément <title> — qui peut être utilisé pour ajouter un titre au document. il peut toutefois se confondre avec l'élément <h1>, qui est utilisé pour ajouter un titre au contenu de votre page dans la balise body — ce terme(title) est également appelé parfois titre de page. Mais ce sont des choses différentes!

  • L'élément <h1> apparait dans la page quand elle est chargée dans le navigateur — generalement il est utilisé qu'une seule fois dans une page , pour marquer le titre du contenu de la page( le titre d'une histoire, ou une actualité, ou quoi que ce soit.).
  • L'élément <title> est une métadonnée qui represente le titre du document HTML global (non le contenu du document).

Apprentissage actif: Inspection d'un exemple simple 

  1. Pour commencer cette pratique, nous vous invitons de telecharger une copie de notre  page-titre-exemple sur notre depôt Github. Pour faire cela,
    1. Copiez et collez le code de la page et dans un nouveau fichier texte dans votre éditeur de code, puis enregistrez-le dans un endroit raisonnable.
    2. Appuyez sur le bouton "Raw" sur la page, choisissez Fichier> Enregistrer la page sous ... dans le menu de votre navigateur, puis choisissez un endroit pour enregistrer le fichier.
  2. Maintenant ouvrez le fichier dans votre navigateur. Vous devriez voir quelque chose comme ça:

    A simple web page with the title set to <title> element, and the <h1> set to <h1> element.It should now be completely obvious where the <h1> content appears, and where the <title> content appears!

  3. You should also try opening the code up in your code editor, editing the contents of these elements, then refreshing the page in your browser. Have some fun with it.

The <title> element contents are also used in other ways. For example, if you try bookmarking the page (Bookmarks > Bookmark This Page, in Firefox), you will see the <title> contents filled in as the suggested bookmark name.

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 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.)

<html lang="en-US">

This is useful in many ways. Your document will be indexed more effectively by search engines if its language is set (allowing it to appear correct 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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