The user settings could have CSS by kind of element (such as <article> or <body>), or by CSS classes, or other attributes (such as ARIA), although stuff such as

The user settings could have CSS by kind of element (such as <article> or <body>), or by CSS classes, or other attributes (such as ARIA), although stuff such as <article> isn't consistently applied anyways. If necessary, there should be some way to specify global CSS rules which are not applied to webpages that have their own CSS (either in the document or defined by the user).