Checking accessibility while I build

I try to check accessibility while I'm building the page, when changing the structure is still easy. Tabbing through a form often tells me something a screenshot can't.

Can I see which field has focus? Does each field have a visible label? If I submit something invalid, can I find the error and understand what to fix? I also check the headings to see whether they describe the page in a sensible order.

Automated checks help me catch mistakes, but I still need to use the page. An error message can be present and correctly associated with a field while giving me no useful idea of what to enter.