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HTML5 First Look
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Understanding the outline algorithm


From:

HTML5 First Look

with James Williamson

Video: Understanding the outline algorithm

One of the more confusing aspects of creating documents with HTML5 is when to use elements like section, aside, article, header, footer, and nav to structure content. While reading the definition of these elements within the specification can certainly help, understanding the algorithm that HTML5 uses to outline documents can add even further clarity to the subject. HTML5 contains many algorithms that instruct user agents how to handle HTML.
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  1. 3m 56s
    1. Welcome
      1m 1s
    2. Using the exercise files
      1m 50s
    3. Who is this course for?
      1m 5s
  2. 21m 12s
    1. Exploring prior standards
      4m 26s
    2. Why do we need HTML5?
      3m 32s
    3. HTML5 timeline
      4m 24s
    4. Current HTML5 support
      4m 25s
    5. What HTML5 is (and what it isn't)
      4m 25s
  3. 27m 49s
    1. HTML5 vs. HTML4
      3m 25s
    2. New structural tags
      6m 1s
    3. New content tags
      4m 7s
    4. New application-focused tags
      5m 32s
    5. Deprecated elements
      4m 28s
    6. API overview
      4m 16s
  4. 22m 29s
    1. Content models
      5m 33s
    2. Understanding the outline algorithm
      3m 21s
    3. The role of ‹div› tags
      4m 20s
    4. Using ID and class attributes
      2m 6s
    5. DOCTYPE declarations
      4m 16s
    6. Character encoding
      2m 53s
  5. 41m 27s
    1. Basic page structure
      3m 40s
    2. Structuring top-level elements
      7m 30s
    3. Structuring interior content
      8m 42s
    4. Building headers
      9m 11s
    5. Checking document outlines
      5m 46s
    6. Ensuring cross-browser structure
      6m 38s
  6. 27m 53s
    1. New input types
      5m 57s
    2. Setting form autofocus
      2m 53s
    3. Using placeholder data
      4m 4s
    4. Marking required fields
      3m 24s
    5. Working with number inputs
      5m 49s
    6. Using date pickers
      5m 46s
  7. 1h 1m
    1. Canvas overview
      6m 21s
    2. Adding canvas content
      8m 58s
    3. Drawing in the canvas environment
      12m 9s
    4. Drag-and-drop API overview
      6m 18s
    5. Offline applications overview
      7m 11s
    6. Video overview
      5m 45s
    7. Encoding video
      8m 23s
    8. Adding video
      5m 58s
  8. 57m 33s
    1. Geolocation API overview
      5m 50s
    2. Web storage API overview
      5m 40s
    3. WebSockets overview
      4m 16s
    4. CSS3 overview
      6m 38s
    5. Enhancing typography with CSS3
      7m 42s
    6. Using @font-face
      7m 11s
    7. Styling HTML5 with CSS3
      10m 23s
    8. Using CSS3 transitions
      9m 53s
  9. 5m 6s
    1. Final thoughts
      3m 49s
    2. Goodbye
      1m 17s

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HTML5 First Look
4h 28m Beginner Aug 23, 2010

Viewers: in countries Watching now:

In HTML5 First Look, author James Williamson introduces the newest HTML specification, providing a high-level overview of HTML5 in its current state, how it differs from HTML 4, the current level of support in various browsers and mobile devices, and how the specification might evolve in the future. Exercise files accompany the course.

Topics include:
  • Understanding the history of HTML5
  • Using new tags
  • Understanding HTML5 semantics
  • Coding ID and class attributes in HTML5
  • Structuring documents
  • Building forms
  • Exploring HTML5 native APIs
  • Encoding and adding HTML5 video
  • Exploring associated technologies such as CSS3
Subjects:
Developer Web Web Design Web Development
Software:
HTML
Author:
James Williamson

Understanding the outline algorithm

One of the more confusing aspects of creating documents with HTML5 is when to use elements like section, aside, article, header, footer, and nav to structure content. While reading the definition of these elements within the specification can certainly help, understanding the algorithm that HTML5 uses to outline documents can add even further clarity to the subject. HTML5 contains many algorithms that instruct user agents how to handle HTML.

Some deal with parsing code, error handling, how to parse badly formed code, and how to handle form submittal. The outline algorithm details how sectioning content should be parsed to build an outline of the current document. Using the rules for the outline algorithm, you can ensure that your page is structured exactly the way you want it to be. This can be useful from a semantic standpoint for accessibility reasons or to make your content easier to syndicate. One of the easiest ways to picture the outline algorithm is to imagine your page as a table of contents.

In a typical table of contents you would list the most important sections as individual items and then list interior sections nested within them. So how does HTML5 decide which parts of your page are used to structure and create your outline? Going back to the content model discussion we had earlier, HTML5 examines how section and heading content is structured within the document to create the outline. The algorithm starts with the body tag and establishes that as the outline's section root.

The parser then walks down the elements in the document in order to establish its structure. Each time a new section is found, another item is added to the outline. If the section contains a heading, the heading is used to name the section. Since sections are treated as containers any new sections created within an existing one are nested in the outline. So what elements create a new section? Again, let's look at the sectioning content model. The article, aside, nav, and section elements will all generate a new section.

Their use in fact should be restricted to only when you intend to create a new section in your document. Now, one thing that can be a little tricky here is the use of headings within sections. The first heading content element in a section is used to define a heading for that section. After that, any additional headings will create new nested headings based on their ranks. It's easier to visualize this than explain it. So let's look at a few examples. So here we have a little bit of HTML markup.

Now, this code would create this outline. Now, what if the second review had subheadings within it? Now let's add a few heading tags here. Okay. So by adding a few headings we get the following outline. Note that the rankings of the headings determine whether they are child or parent nodes within the outline. Headings with the same or greater rankings create a new section whereas headings with a lower ranking create a nested section.

If you take a moment to write a brief outline of your page before you begin coding it, you'll be well on your way to understanding your pages initial structure and the elements you should use to construct it.

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