SEO is undoubtedly about the quality of your content. If you provide quality content to your visitors your will rank higher in search engines whether your HTML and CSS coding is correct or not. What really matters is your end result. If your website renders fine to your visitor then you are safe with Google.
But its not this simple, Google does consider bounce rate of your visitors while ranking your website and it has been seen that bounce rate is pretty low if you are not using valid html, because the outlook of your website does tell the visitor whether you are professional in your field or not. So, it very important to learn html. And its even more important to learn css so that you can create your own pagelayouts using div tag . This benefit you as you will gain trust of your visitors