SVG-AAM: Use graphics roles, fix unclear requirements

[AmeliaBR]

Compiled version in RawGit

• Lots of clean-up of unclear text, with a few normative changes based on discussions over the past few months.
• Changes the role mapping table to use the new ARIA Graphics roles as defaults.

Addresses issues #134, #136, and #137, along with various issues and actions on Tracker.

Comments welcome on these changes, or on any of the outstanding Editor's Notes, preferably before the next SVG Accessibility Task Force teleconference on 25 May 2016.

[AmeliaBR]

Comments from Fred (@fesch):

• Part 1, with Amelia's reply
• Part 2, with Amelia's reply
• Part 3, & Amelia's reply

Typos & wording fixes have been integrated in this branch.

The main outstanding debate is when should elements that are visually hidden with styles count as "aria-hidden" and excluded from the accessibility tree. The current draft excludes all elements:

• that are not rendered due to SVG semantics. This won't change.

• that are not rendered due to display: none. This won't change by default; we can discuss whether it should be possible for an authors to over-ride this with aria-hidden="false"; however, we'd need commitments from user agents to go that path, as this is currently poorly supported in HTML.

• that are not visible (via visibility: hidden) and/or not painted (via fill: none and stroke: none and marker: none on elements that are normally filled & stroked & allow markers) and are not sensitive to pointer events based on the pointer-events property and have not been explicitly included with aria-hidden="false".

  This could be simplified to only use visibility and pointer-events, although pointer-events itself relies on fill and stroke. It would risk falsely including extraneous elements that aren't currently painted.

Other styles and attributes that can currently cause an element to be invisible (e.g., zero opacity, transparent fill, zero height/width, clipping, positioning off screen) are not assessed when determining if an element is "hidden" based on SVG/CSS semantics. An author would need to explicitly add aria-hidden="true" to ensure that elements made invisible with these styles are ignored by assistive tech. Conversely, if these styles were used to express a valid zero data value (like a zero-height bar on a bar chart), the element would still be included even though it doesn't have any corresponding painted pixels on screen.

Because elements can be invisible but included in these cases, it perhaps makes sense to allow element invisible because of fill and stroke to also be included.

[AmeliaBR]

Resolutions from 25th May telcon of the SVG Accessibility Task Force:

• Change SVG-AAM so that fill:none and stroke:none are not considered native semantics for hiding an element
• Accept Amelia's edits to SVG-AAM pending completion of [those changes]