We were supposed to be focused on semantics in HTML, then in HTML5 which failed at semantics but blew the browser open for programmatic trickery. But nearly every shopping site has checkboxes that require a browser refresh, or don't let you make multiple selections. Why? WHY WHY?

I don't understand the question. Out of HTML5 came the ability to script and expand elements via APIs, moving browsers from a document object model to a programmatic one. If you're asking about checkboxes, Amazon has the problem, eBay handles it differently and better IMO.

A checkbox should not force a browser refresh, but allow the user to make multiple selections and then input the data in one fell swoop rather than every single query. It's UX FAIL of the worst kind, and breaks expected behavior. Not cool.