[css-text-decor] Control the line height of text containing emphasis marks

[xfq]

Many fonts (most Simplified/Traditional Chinese fonts and quite a few Japanese fonts) tend to increase line height when using text-emphasis-style, which usually leads to undesirable results.

To address this issue, we may need a new CSS property to:

1. force the browser to select a font that displays emphasis marks correctly without widening line spacing, and/or
2. manage the spacing/proximity of the emphasis marks alongside the text (kind of like text-underline-offset)

I'm not sure if font formats can offer information about the appropriate position of emphasis marks. If so, the UA should use such font-based information when available.

Currently, we already have this text in CSS Text Decoration:

the UA may opt to use a font known to be good for emphasis marks, or the marks may instead be synthesized by the UA.

However, this is insufficient because it is only a MAY, and there is no way for authors to enforce the behavior.

See more background information in w3c/jlreq#446

/cc @bobbytung