Back in the 90's books would always talk about using body and head tags. Do those tags even matter anymore?I've added scripts and style tags and the browsers accept them without any tags. 
Do ,  and  tags even matter anymore?
html;web development;development
If you don't use proper HTML, including , , , you can't validate a page, so no easy way to spot problems with the actual page coding. I'm going to guess that invalid HTML also significantly affects SEO, to the extent that matters.