Nothing about Gatsby or React by itself gets you non-validating HTML. The article has some W3C validation errors, but they seem to be caused by a few CSS errors they made themselves, the choice by their chosen CSS framework to put <style> tags for components directly in the page body next to their component elements, and their chosen code highlighter's choice to use <div> tags inside of <pre> tags. I find it surprising those two tools do that, though I wouldn't describe those issues as having anything to do with the rest of the ecosystem. (I have a Gatsby site using the component CSS library Styled-JSX and the code highlighter PrismJS which don't have those issues, and the site only has a few validation warnings that are purely from my own html. Apparently <section> tags are expected to have headers.)