I recommend you look into user stylesheets, see: https://superuser.com/questions/560539/how-can-i-force-my-css-styles-when-i-visit-a-website
You can change your browsers user stylesheet, but the easiest method is to install the Stylish extension on Chrome or Firefox and add this:
* {
user-select: text !IMPORTANT;
}
Stylish also allows you to disable the sheet on certain websites, or have per-site custom styles.
This may break things on certain websites, so I recommend you either configure Stylish to not apply the stylesheet on those sites, or you use a more specific selector rather than *
.
A final caveat, this can be defeated if the website uses !IMPORTANT
, at which point you may need to look into more involved solutions, such as running JavaScript on page load to remove the offending CSS rules.