Minimum used line-height of text-entry widget doesn't match implementations

[jfkthame]

See https://html.spec.whatwg.org/#the-input-element-as-a-text-entry-widget:

Additionally, the 'line-height' property, if it has a computed value equivalent to a value that is less than 1.0, must have a used value of 1.0.

This rule is implemented by Gecko. But it appears that Webkit and Blink implement a different behavior, sites depend on that behavior, and as a result exhibit poor rendering in Gecko. See webcompat/web-bugs#47819 for an example.

I believe the Webkit/Blink result is preferable, and would like to modify Gecko to behave similarly.

Suggested new text to replace the sentence above:

Additionally, the used value of the 'line-height' property must be no smaller than the used value would be for 'line-height: normal'.

The idea here -- which Webkit and Blink appear to have adopted, despite what the spec currently says -- is that a used line-height of 1.0 is often too small and will result in unwanted clipping. This is what we see happening in Gecko. Given that clamping the minimum used line-height to normal rather than 1.0 appears to be web-compatible, I think we should simply change the spec (and Gecko) to reflect this.

[zcorpan]

This seems OK to me. I can change this and update test (or write a new test if there isn't one) probably next week.

[zcorpan]

It has been a long week... but I've now written tests: web-platform-tests/wpt#32684