Intent to ship: CSS `tab-size` property

[Jonathan Kew]

As of Firefox 91, I intend to enable the CSS `tab-size` property by
default on all platforms.

This property has been shipping in Webkit and Blink browsers for a long
time.

In Gecko, we have had a prefixed `-moz-tab-size` for many years. The
bug[1] to unprefix and expose it under the standard `tab-size` name was
filed 9 years ago. However, we've been reluctant to do so because our
feature did not entirely conform to the spec[2].

In [3], which is about to land in Nightly, I am fixing the behavior of
-moz-tab-size to implement the current spec requirements. With this
fixed, we can at last proceed to unprefix (-moz-)tab-size.

(There's no corresponding Intent to Implement for this, as the feature
has mostly been implemented "forever". We're just fixing bugs in its
behavior and exposing it under the standardized name.)

The -moz-prefixed name will remain available as an alias, at least for
the time being.

JK

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=737785
[2] https://drafts.csswg.org/css-text-3/#tab-size-property
[3] https://bugzilla.mozilla.org/show_bug.cgi?id=1308113