`all-small-caps` value for `font-variant` CSS attribute is not supported

[windymilla]

The all-small-caps value for font-variant is listed in CSS Fonts Module Level 3 which is at REC status. However, using it gives messages such as

ERROR:cssutils:Property: Invalid value for "Added Properties for Project Gutenberg/CSS Level 2.1" property: all-small-caps [100:15: font-variant]

[eshellman]

Thanks, we can add it to our CSSutils config.

Currently we have 'font-variant': r'normal|small-caps|inherit|initial'

Anything else we should add?

[windymilla]

There are several other possibilities, but none of them look useful for us, so just all-small-caps please.