I don't know about "must" know html/css, but to get beyond a certain level of competency in designing web pages, to be able to make designs that are easily tran

I don't know about "must" know html/css, but to get beyond a certain level of competency in designing web pages, to be able to make designs that are easily transferable to code, a knowledge of html/css will come into play. I've worked with designers with and without this knowledge before, as well as designers familiar with css, but not html, and I can say that working with designers that knew html/css is far easier. Much less back and forth. Also, the ones without the knowledge get annoyed when told their designs don't translate well to code. HTML/CSS does not take that much effort to learn. Just put in the effort and at least pick up the basics. Read a book on it. You don't need to know the ins and outs, but knowing the basics will help you design sites. And maybe you will find it interesting, and learn all the nitty gritty too. It will make you a better web designer.