Make the note less ambiguous

[SelenIT]

The specification actually doesn't require border-style and/or border-width for border-image to be rendered (w3c/csswg-drafts#655 (comment)). However, some browsers indeed would not render it without them (although this is technically a bug and is being fixed), so specifying them is a reasonable practical requirement at the moment (and providing fallbacks is a good practice anyway).

Issue number that this PR fixes (if any). For example: 'Fixes #987654321'

What was wrong/why is this fix needed? (quick summary only)

Anything else that could help us review it

[github-actions]

Preview URLs

• https://pr8912.content.dev.mdn.mozit.cloud/en-US/docs/Web/CSS/border-image

Flaws

URL: /en-US/docs/Web/CSS/border-image
Title: border-image
on GitHub
Flaw count: 5

• macros:
  • /en-US/docs/Web/CSS/conic-gradient() redirects to /en-US/docs/Web/CSS/gradient/conic-gradient()
  • /en-US/docs/Web/CSS/linear-gradient() redirects to /en-US/docs/Web/CSS/gradient/linear-gradient()
  • /en-US/docs/Web/CSS/repeating-linear-gradient() redirects to /en-US/docs/Web/CSS/gradient/repeating-linear-gradient()
  • /en-US/docs/Web/CSS/radial-gradient() redirects to /en-US/docs/Web/CSS/gradient/radial-gradient()
  • /en-US/docs/Web/CSS/repeating-radial-gradient() redirects to /en-US/docs/Web/CSS/gradient/repeating-radial-gradient()

External URLs

URL: /en-US/docs/Web/CSS/border-image
Title: border-image
on GitHub

No new external URLs

[rachelandrew]

Thank you!