The CSS box model for sure. Border box just makes so much more sense than content box (who seriously counts the width of a border and padding as separate from t

The CSS box model for sure. Border box just makes so much more sense than content box (who seriously counts the width of a border and padding as separate from the container itself, in real life or otherwise?) that I'd put it as default from the start, like IE did all along. I'd probably also scrap the attempts to migrate to XHTML, and go ahead with HTML 5's additions back in the early 00s. Things like the new semantic block elements, new input types, picture element, etc are far more useful to developers than an XML based HTML dialect were, and don't sacrifice ease of use for some abstract ideal. Finally, I'd have Flexbox and Grid added early on, so that the hellscape of floats and table layouts didn't have to come to fruition. They just make so much more sense really.