Looking for tools to correct, optimize and improve compatibility of a website

Question

I'm interested in a list of good, free online resources to check the correctness of html and css code, optimize the user experience (loading times etc.) and improve the cross-browser compatibility.

Ideally, I think that this thread would work best if everyone sticks to one link (and explanation of what the tool does and why it is good) per answer, so that the upvotes can really reflect our opinion on the best resources.

Answer 1

Use GT Metrix , it's very Popular :- https://googleweblight.com/i?u=https://gtmetrix.com/&grqid=N44cI1Tg&hl=en-IN