For example, using <center> and <br> design-wise is deprecated, font is not good, too much padding on content (not matching header size) also too much of <ul>. And also, many common things what should be in <header> are simply not there. And. no language specified in main <html> tag. That are just few things what I noticed when I pressed View source code on SFML Game Jam site index.
<html> and <br> are html5 compliant. Language and charset can be defined in meta tags.
Are you talking about <header> html5 tag or <head> tag ? Because nothing is mandatory in <header> tag, it just do indicate the title part of a logically linked block (like the website itself, an article, etc.)
Yes some metas are missing from <head> to be googlebot-compliant, but since people that get on SFML Jam website probably come from here (meaning this forum), it's not an absolute obligation to have them (but can be added whenever possible)