[jaster]

Not sure if that is what you are asking, but Zotero has plugins for saving web page directly from the browser: https://www.zotero.org/download/connectors

I use it extensively with FFox.

Regarding highlight tool I can't say, I did not look for the feature (I usually take my notes in a separate Note item attached to the article)

[fudged71]

Is a link saved as an offline web format with fulltext, or just as a link?

[gildas]

It uses SingleFile under the hood so it saves a full offline page in HTML.