Browsers have been backwards compatible since the first one. Backwards compatibility is one of the mainstays of the Internet. As long as browsers continue to support deprecated and obsoleted tags and attributes, it makes no difference which you choose, attribute or CSS. Forward compatibility?? Who coined that term?
Maybe some (or many) of the deprecated and obsoleted tags and attributes don’t render well on every browser. So what? They never have so far. Not to mention the fact that even the tags, attributes and CSS that are current are not rendered identically on all browsers.
In summary, clean up the browsers and then you won’t have to worry so much about deprecated and obsoleted tags and attributes.