Et Tu, Grammarly?

I don’t think that ‘hostile’ is really fair in this case, when ‘insufficiently competent’ will do (albeit at the cost of more syllables).

I am not a fan of Grammarly or their technical model, but I don think it’s fair to attribute malice when it is adequately explained by stupidity.

It’s been a long time since I did any front-end work: should both Grammarly’s extension and your own code use namespaced property names?

yep, it's in Grammarly's interest to namespace or scope their CSS in a way that doesn't conflict. Not doing it adequately goes both ways, website CSS could break their extension, or their extension could break the website.

We tried applying cunningham’s law widely and it created disastrous incentives. It’s better to assume profitable yet destructive incompetence is malice.

Any sufficiently advanced stupidity is indistinguishable from malice.

Unfortunately browsers don't really provide good solutions for extensions that need to inject or change sites. Look at Google's owner in-browser translate extension, its DOM manipulation breaks many interactive apps as well. There are no tools available in browsers for it to not need to do that.