I respectfully disagree. I could care less what other HTML renderers do. It is bad form to use certain format settings inside the CSS and then hope the renderer will correct your garbage. But then again I spent a quarter century as a technical writer so that probably explains why I disagree. Every time I inherited another TWs book I would typically have to spend a lot of time cleaning up formatting code so it would print/display correctly. This was especially important in the early years when printing was such a PITA. I'm talking professional printing, as in send your book to a professional print shop. And yes, if your files contain garbage and a renderer pukes out garbage it is your fault, not the fault of the renderer.

My philosophy is to make sure the formatting, and everything else, is as clean as possible. Sure the ePub and HTML renderers might be forgiving and turn out decent eBooks from your files, but occasionally you are going to come up against a renderer that doesn't. So clean is important. It is also important for when the book gets converted to other formats whether digital or print. And yes, if your book contains garbage and the renderer pukes out garbage, it is your fault, not the fault of the renderer.