This is just a minor issue.

There is an inconsistency in the way some "enumerated" type properties are
handled. Typically they are checked and if a value outside the spec is
encountered a suitable error is raised, e.g. font-stretch has the allowed values
"normal | wider | narrower | ultra-condensed | extra-condensed | condensed |
semi-condensed | semi-expanded | expanded | extra-expanded | ultra-expanded |
inherit" and if anything else is put into the property value the user will get
an error. However, I noticed that this is not the case for font-style whose
allowed values are "normal | italic | oblique | backslant | inherit" or
font-weight (allowed values: "normal | bold | bolder | lighter | 100 | 200 | 300
| 400 | 500 | 600 | 700 | 800 | 900 | inherit"). These properties accept non
conformant input without rasing an error or a warning.