Follow-up to parts 1 and 2.
This section covers events, actions, hit testing, relative coordinates,
text bounds, and iframes - filling in many of the additional complexity
that was glossed over in parts 1 and 2.
It also adds a few more links to "How Chrome Accessibility Works" from
other Markdown pages.
Bug: None
Change-Id: Ic1037d349555a981d16051526acde36247e32c0f
AX-Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3137196
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#917367}
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