The outlinemask attribute

The h1, h2, h3, h4, h5, or h6 elements may have the outlinemask content attribute set. The outlinemask attribute is a boolean attribute. When specified on one or more h1 to h6 elements,it indicates that the element is not to be included in the document outline. User agents should not include the h1 to h6 elements or their content in the document outline if the outlinemask attribute is specified.

The outlinemask IDL attribute must reflect the content attribute of the same name.

The ARIA role mapping for a h1 to h6 element that has an outlinemask attribute is the same as a h1 to h6 element without an outlinemask attribute set.

User agents MAY provide an indication that a heading is a subheading when the attribute is set.

Here are some examples of valid use of the outlinemask attribute. In each case, the subline content represents the text that is a subheading, subtitle, tagline or byline of a heading. The subline content would not be used as a heading in an application extracting heading data and ignoring subheadings.

 <h1>The reality dysfunction</h1>
<h2 outlinemask>Space is not the only void</h2>

 <h1>Dr. Strangelove</h1>
<h2 outlinemask>Or: How I Learned to Stop Worrying and Love the Bomb</h2>

The reasons for using outlinemask attribute in these examples is to mask the h2 element from the outline algorithm and convey the semantics of the h2 element (as a secondary title) .

Assistive technology may convey the semantic of the outlinemask attribute instead of the heading it is on. Thus for an assistive technology user the first example above could be announced as:

"heading level 1 The reality dysfunction subheading Space is not the only void"

Or the assistive technology may convey both the heading and outlinemask semantics.

"heading level 1 The reality dysfunction subheading, heading level 2 Space is not the only void"

Or the assistive technology may convey only the heading semantics.

"heading level 1 The reality dysfunction, heading level 2 Space is not the only void"

Non-conforming features

The element in the following list is an addition to those defined as obsolete in HTML5, is entirely obsolete, and must not be used by authors:

To identify sub headings Use the outlinemask attribute instead.