[css-highlight-api] Support for some font styles and decorations

I'm currently building a new code editor for the web that uses the CSS Custom Highlight API for syntax highlighting, with the idea being that it would simplify things significantly while at the same time improve performance.

But, some basic forms of font styles and decoration (italic, bold, underline, strikethrough) are not supported by this API. As I understand it only properties that can be applied efficiently are supported, but arguably a subset of the font-style and font-decoration properties can be applied efficiently in some scenarios, even if this capability were limited to monospaced fonts it would unconstraint my use case, which would be amazing.

As far as I know the folks at VS Code are also exploring using highlights for syntax highlighting, which would potentially enable optimized syntax highlighting for millions of developers worldwide, so it'd be extra nice if highlights could be used for 100% of normal grammar-based syntax highlighting, rather than the current 99%.

Start by reading the CSS Custom Highlight API and its current restrictions on efficiently applied properties. Compare the API's supported styling model with the issue's italic, bold, underline, and strikethrough examples. Done means the proposal has a settled scope for supporting the requested font styles or decorations.