[css-fonts] Display gaiji as text if glyphs are supported

Authors (particularly in east-asian languages) tend to use images to represent rare and uncommon characters. These images are commonly called gaiji images. Use of gaiji in content causes several problems which include:

• Image may look blurry at larger font sizes
• Alignment of image baseline with the text baseline
• Font in image may not match the base text

Authors don't trust all browsers and readers to represent these glyphs correctly, so even though it may work in one application they will continue to use the image as long as the lowest common denominator does not support the glyph.

In order to break this deadlock, a property could be added that allows for a UA to display the alt-text of an image if those glyphs are supported by the UA. If the glyphs are not supported, the UA will display the image as normal.

Currently, it seems an appropriate place to add this could be as a new keyword to "content".

https://www.w3.org/TR/css-content-3/#content-property

Start by reading the CSS Content Level 3 content-property section linked in the issue, then review the five-comment discussion for existing constraints. Define the proposed keyword's syntax and fallback behavior for gaiji images, and consider the specification changes needed to make the behavior implementable and testable.