This tip has been brought with version 2.4.0 of The SEO Framework.
Invalid HTML doesn’t affect your site’s SEO value. However, valid HTML is considered best practice, and with a good reason.
The good reason for valid HTML is that you can be certain that your website (almost) looks the same on every browser, so you’re certain that the visitor will see your website as you intended.
However, it doesn’t affect your site’s SEO value because Search Engines want to put out the best content for your search, not the most ‘valid’ content.
With so many “invalid” websites out there, Search Engines try to read the HTML as many web browsers would: by fixing common mistakes.
Still using HTML4 or xHTML? Try to find a way to upgrade to HTML5, its semantic elements are very SEO friendly.