HTML Guidelines

Header

Header usage should be hierarchical within page views Reference link: Standardize classnames and layout to facilitate theming

Headers should be hierarchical, with two or more <h3> under each <h2> and so forth. However an exception applied to front-page where multiple <h2> are allowed for displaying the page's items (eg: list of courses, list of categories, etc).
<h2> is the top-level header of the page content/central column. There should be only one <h2> (except for front-page where multiple <h2> are allowed).
Headers should not be used for notification. Notification classes should be used for notification.
For H1 and H2 elements, no additional classes should be added. For H3 and below, some classes that describe the purpose of that heading such as sectionname, and others that provide functionality, such as dimmed_text, may be needed. Blanket and vague classnames should be eliminated (main, header, title, mdl-align, etc).
We shouldn't be wrapping a heading in a div just to allow it to be styled. This complicates the DOM, and defeats the purpose of any effort to standardize the appearance of headings across the page.

Not this:

  <h2 class="main">This is the top-level header</h2>
  <p>A paragraph of content.</p>
  <h2 class="main">These is your score on XX activity.</h2>
   <p>A paragraph of content.</p>
  <h2 class="main">You must make a selection to proceed.</h2>
  <input type="submit" value="Generic submit button">

This:

  <h2>This is the top-level header</h2>
  <p>This paragraph gives additional information and explains what's to come.</p>
  <h3>Content sub-header</h3>
  <p>More paragraph content content content</p>
  <h3>Content sub-header</h3>
  <p>More paragraph content content content</p>
  <div class="notifyproblem">You must make a selection to proceed.</div>

Block

Blocks should consist of the following:

Block's name should start at level 2
Block's alignment should used the definition in theme, unless it is forcely modified through Tinymce which is using inline styling.

Activity page

The display for the activity's content should contained the following:

Course name with level 1 header
Activity name with level 2 header
Content header should start at level 3 and lower for its sub-content. Although Tinymce allowed the use of level 1 and 2, it should be avoided.
Use theme alignment for displaying the content.
Content styling such as: alignment, font-size, font-family should inherit from theme.

Update in core API:

As part of removing '.main' class from header tag, we have updated the following functions in core outputrenderer API file:

Heading

For this function we changed the default value for the classes parameter value from 'main' to null. heading($text, $level = 2, $classes = 'main', $id = null) To heading($text, $level = 2, $classes = null, $id = null)

Heading with help

For this function we added two additional parameters to set the heading level and classname. The level parameter value is defaulted to 2 and classnames is defaulted to null. heading_with_help($text, $helpidentifier, $component = 'moodle', $icon = , $iconalt = ) To heading_with_help($text, $helpidentifier, $component = 'moodle', $icon = , $iconalt = , $level = 2, $classnames = null)

The detailed of changes for the above functions is reported in MDL-41438.

Documentation