Fix issue 14318: explain better about invalid custom CSS properties

[wbamberg]

Fixes #14318.

I took the opportunity to do a bit more cleanup here and expand on the difference between normal and custom properties in this respect.

I deleted the BCD table because I think there's some emerging consensus that we don't want to have these things outside reference pages. (mdn/yari#5971 (comment)).

[github-actions]

Preview URLs

• https://pr15736.content.dev.mdn.mozit.cloud/en-US/docs/Web/CSS/Using_CSS_custom_properties

Flaws

URL: /en-US/docs/Web/CSS/Using_CSS_custom_properties
Title: Using CSS custom properties (variables)
on GitHub
Flaw count: 2

• broken_links:
  • Can't resolve /en-US/docs/Web/CSS/var()
  • Can't resolve /en-US/docs/Web/CSS/var()

External URLs

URL: /en-US/docs/Web/CSS/Using_CSS_custom_properties
Title: Using CSS custom properties (variables)
on GitHub

No new external URLs