It would come with options for the user to select font/size/colors for various well-defined elements like head-lines, new links, previously visit links, externa

It would come with options for the user to select font/size/colors for various well-defined elements like head-lines, new links, previously visit links, external links. It would be easy and convenient for the user to configure their preferred formatting of the text (width, line break behaviour, justification). It would not allow client-side execution of any kind, it would not allow any kind of styling of the content, only describe what the content is, not how it should be presented.