I wish there was a platform where I could express the UI purely in terms of content, structure, and behavior, and let the design be supplied by the platform, wi

I wish there was a platform where I could express the UI purely in terms of content, structure, and behavior, and let the design be supplied by the platform, with the ability for the user to override it. Then someone else could deal with fickle fashions while we application developers focus on what matters. I don't even want to have to provide a default design, e.g. a Bootstrap stylesheet. But of course, if I use pure browser defaults, it looks like something from the early 90s. Maybe the HTML5 doctype should have told the browsers to go with fashionable defaults in addition to turning off quirks mode.