[DevTools] Page inspector

[chrisdavidmills]

Acceptance criteria

1. • [x] The listed features are documented sufficiently on MDN
2. • [x] BCD is updated - HW: not applicable
3. • [x] Interactive example and data repos are updated if appropriate - HW: not applicable
4. • [x] The content has been reviewed as needed
5. • [x] Set bugs to dev-doc-complete
6. • [x] Add entry to Firefox release notes if feature is enabled in release
7. • [x] Add entry to Firefox experimental features page if feature is not yet enabled in release - HW: Not applicable (enabled in release by default)

Related Gecko bugs

1. • [x] Add functionality to display tabbing order overlay — nice little addition to the Firefox accessibility inspector. Needs documenting somewhere appropriate.

[hamishwillee]

Essentially feature is a checkbox you can select to add numbered highlight showing the tab-order - the order that you would scroll through links/buttons if you press tab. This is useful because we expect pages to have a logical tab order for people who can't use mouse, and this lets us immediately see that the order is sensible.

The feature is not dynamic; if there is a closed panel when you select the checkbox, the items will not automatically update once you have

Changes:

• • [x] Add new section Firefox Developer Tools > Accessibility Inspector > Show tabbing order with docs on feature [This is most important update - rest of things link to this]
    • • [x] Clean up out of date images in Accessibility Inspector
• • [x] Accessibility > HTML > UI controls - Added note linking to accessibility inspector "show tabbing order" tool.
  • • [x] Cross Browser Testing > Handling common accessibility problems > Using native keyboard accessibility - added note linking to the tab overlay too. Also minor update to note that the importance of tab testing is both access to functionality AND flow.
• • [x] Page Inspector > HowTo Add link to Accessibility Inspector into Howto List
• • [x] Release notes > Developer Tools
• • [x] Update bug with doc info and dev-docs complete.

@chrisdavidmills This one ready for your review

[chrisdavidmills]

Reviewed, small edits made, but very happy with this; reads really nicely.

Thanks @hamishwillee !