&gt; The overhead for running a few select extensions, compared to native code, would be minimal.<p>Only if the browser is designed that way. If the browser has only global settings implementing per-site handling can impose the need of extension reimplementing half of the browser (in extreme case, see how Google made their &quot;extension&quot; of the IE which effectively installed the 90% of the Chrome inside of IE).<p>And you have to trust extension authors, they can do their own things.<p>Old Opera had what I needed built in, it worked without the need of even enabling javascript subsystem. I don&#x27;t know of any other browser able to work that way.