css with inline styles and important tags everywhere
It's pretty difficult to achieve CSS isolation. That is, it's essentially impossible to get CSS to only selectively apply to a certain element, free from the influence of other things on the page.
Normally, that's not a problem, because you control your own website. Sometimes this assumption gets challenged a bit when advertisers decide to throw a coup on your own property, but in the end you still have the veto power.
Extensions however can't control other websites, and it's quite difficult to inject content onto a page without the fear of interference by the embedding page.
The typical answer to isolation is the judicious use of iframes, which is in many ways an untenable proposition. Iframes are heavy, slow and have a tendency of ripping open the wormhole to the late '90s our superhero forefathers so valiantly sought to close once and for all.
In recent history, there have been whispers amongs the tree-folk of a new entity lurking the living forests during the night. The Shadow DOM is its name.