This CSS property allows input carets (that blinking input cursor you see in text fields), to be given a custom color. This was implemented in Firefox 53, and it was such a minor feature that no one ever missed it, but I don't see any harm in implementing this.
https://bugzilla.mozilla.org/show_bug.cgi?id=1063162
Even though percentages are already treated as floats internally by the style system for computation purposes, you have to go out of your way to stop them from being read back out as percentages. What I do here amounts to storing the percentage token in the "wrong" container, the one normally used for floats. This allows a value that was read in as a percentage to be read back out as something else, which is normally prevented by the design of the style system.
This preliminary step allows percentages to be computed and display correctly,
but unfortunately it fails a test after changing VARIANT_HN to VARIANT_HPN because that allows values to be serialized as percentages. However, not doing this means percentages are rejected as valid values for the user to input. The way the style system is setup makes it hard to change this for opacity without changing it for everything else, especially since some code-saving speed hacks in Bug 636029 and Bug 441367 that make a lot of assumptions about this stuff very rigid.
This is just a clean port of 1322191 and follow-up 1325970. It really seems to add create a new way to access existing code relating to block formatting and floating elements rather than implementing new functionality, and it is mercifully straightforwards.
* Prevent canvas custom content from becoming NAC when reframing the root element
* Add an API to get computed style values through an AnonymousContent object
Tag #1375
List of relevant patches applied:
1398537 part 2 - [css-multicol] Implement percentages for 'column-gap' (Gecko part).
1434478 part 1 - [css-grid] Stop back-computing percentage grid gaps when the percentage basis is indefinite. Treat them as zero sized instead.
1434478 part 2 - Stop back-computing percentage padding/margin when the percentage basis is indefinite. Treat them as zero sized instead.
1434478 part 3 - Remove IntrinsicISizeOffsetData::hPctPadding/hPctMargin members since they are now unused.
1434478 part 4 - Factor out constants like NS_UNCONSTRAINEDSIZE so they can be used in headers without needing nsIFrame.h (idempotent patch).
1434478 part 5 - Create nsLayoutUtils::ResolveToLength for resolving CSS <length-percentage> (idempotent patch).
1434478 part 6 - Propagate a percentage basis to nsIFrame::IntrinsicISizeOffsets for resolving padding/margin.
This is needed only for CSS Grid since in other cases we're only using IntrinsicISizeOffsets in the inline-axis and the percentage basis is always indefinite for *intrinsic sizing*. When calculating the intrinsic size of grid items in the grid container's block axis however, we do have a definite size for the grid area in the inline-axis and it should be used per: https://drafts.csswg.org/css-grid/#algo-overview "2. Next, the track sizing algorithm resolves the sizes of the grid rows, using the grid column sizes calculated in the previous step." (Percentage padding/margin for grid items is always resolved against the grid area's inline-size nowadays.)
