Backplate should be live and released starting in FF73, so we'll need to change the preference accordingly and uplift.

Hooking this up to where this got implemented. Also moving to layout, as this isn't a bug relating to the implementation of the preference system. :-)

Comment on attachment 9120543 [details]
Bug 1608889: Flip backplate preference to true to ship in 73.

Beta/Release Uplift Approval Request

• User impact if declined: Users will not experience readability backplate by default when using high-contrast mode. Users will continue to see no background images render, and may have a sub-optimal browsing experience compared to using HCM in Edge.
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: No
• Needs manual test from QE?: Yes
• If yes, steps to reproduce: 1. Navigate to about:config and verify 'permitBackplate' displays as "true" by default

2. Navigate to cnn.com and verify images are displayed, and text overlapping images is "backplated" with a white background.

• List of other uplifts needed: None
• Risk to taking this patch: Medium
• Why is the change risky/not risky? (and alternatives if risky): Though backplate has been tested pretty thoroughly by the accessibility team (and by my written tests), there is still a possibility there are problems we have not uncovered. This is a large change to how users experience HCM, and though we believe it will lead to a better overall browsing experience, we wouldn't say it comes without risk. To mitigate the possible risks, though, we've elected to keep the preference (permitBackplate) and change its default value to "true" rather than remove the pref entirely. If large issues appear that make backplate unusable, we can uplift a patch to change the default of this pref back to "false".
• String changes made/needed: N/A

Comment on attachment 9120543 [details]
Bug 1608889: Flip backplate preference to true to ship in 73.

Enables the HCM backplate by default for Fx73. Approved for 73.0b5 \m/

https://hg.mozilla.org/releases/mozilla-beta/rev/1aed30219c61

https://hg.mozilla.org/mozilla-central/rev/712292576d96

I have verified that the pref browser.display.permit_backplate is correctly set to true in Beta 73.0b5 and Nightly 74.0a1, and the background images are properly visualized.

This was tested on Windows 10 x64, macOS 10.13 and Ubuntu 18.04 x64. Note that on OS X, I didn't see any differences (pref set on true vs false) with HCM applied, the text and images were correctly displayed in both cases.