Repro steps:

• Go to https://en.wikipedia.org/wiki/Main_Page.
• A large notice should appear at the top.
  • If not, clear all cookies for the site, refresh the page.
• In "My filters" pane, manually enter the following cosmetic filter: en.wikipedia.org###centralNotice.
• Refresh the above web page.
• Result: the large notice is still visible.

Cause:

The DOM element #centralNotice has a style property display: block !important, and that CSS rule is part of a style tag which is inserted after uBO has inserted its own style which contains site-specific cosmetic filters -- so essentially uBO's CSS rules are overriden by the page's own rules.

Solution: prepend all inserted CSS rules for cosmetic filtering purpose with html, to augment one notch the specificity of uBO's CSS rules.