I've found a topic about this but it's old and long so I thought I'd ask here. What is the general consensus now, is zp in favor of html or xhtml? Are there plans to switch to html or xhtml is the way to go?

Zenphoto currently generates XHTML 1.0 transitional. There are no plans to change that as it is of minor importance until HTML5 is actually a standard and supported by all browsers (or if you will, until all possible users use browsers that do).