Previously we were overly generous in what we accepted for `display`.
For example, we accepted:
- Multiple outside or inside identifiers
- Unrecognized identifiers
- Non-identifier tokens
Now, we reject all these cases. This makes us stop accepting the
`display: block math;` declaration in `MathML/Default.css`, and so the
one layout test we have containing MathML elements has changed to
reflect that.
3b0980c89f