Specify handling of fallback/sub DOM content for <canvas>

[rahimabdi]

Subissue from this closed issue: #3.

The <canvas> element is an embedded content element which supports "fallback content", in the form of an author-provided sub DOM, to make it accessible.

Currently, <canvas> maps primarily to image/graphic accessibility APIs: https://w3c.github.io/html-aam/#el-canvas. The spec should add more details around this element's mapping of fallback/sub DOM content.

Some additional supporting details:

• Part of this work may involve adding a separate section for handling embedded content generally (e.g., <canvas>, <audio>, <video>, <picture>). However, <canvas> mapping could align with current html-aam prose for other embedded content elements; for example, see mapping for <audio>/<video> which states:

"If the controls attribute is present, UI controls (e.g., play, volume) are exposed as children of the audio element in the accessibility tree, and mapped as appropriate for the type of control (e.g., button or slider).")

• <canvas> differs from other embedded content elements insofar that it's fallback content is always conveyed to keyboard/assistive technology users (not just when the element is unsupported). Since the fallback content represents the accessible alternative (everything inside the <canvas>'s opening and closing tags), it may be worth mentioning that an appropriately-placed closing tag (</canvas>) is required.