David75, thanks for the clarification. If this is a testing/learning environment, are you able to run through a proxy? I'm asking because that should allow more options for injecting this. Second, what level are the end-users and what types of sites will they usually access? I'm thinking of ways to reduce the code required on the page, but that saving might reduce robustness.
Fotiman, thanks for joining in. Is it possible to reduce that code by using an element we don't see often, combined with embedded rather than inline style? For example, <s>, with
text-decoration: none !important;
Would depend on the sites and whether the end-users would notice if the style hadn't been applied - but at least something like <s> provides immediate visual feedback. Also, in ff it is possible to add the css in usercontent.css - could the .js be added via user.js?
In terms of extensions didn't some of the Greasemonkey-type allow you to do what is desired here? After a quick browse at moz, most seem discontinued, but wasn't one specifically designed to make it easy to regex to manipulate the DOM? Can anyone else recall it - and perhaps a name (or similar for webkit/chrome etc)?