Validation of HTML & CSS Markup
Properly written HTML & CSS will render better, on more browsers, and faster than HTML & CSS with errors. Also, properly written HTML & CSS is more likely to be future-proof (backward compatible with future standards and future web browsers).
To validate the code of your store we're using the W3 validator. The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web. Founded in 1994 and currently led by Tim Berners-Lee, the consortium is made up of member organizations that maintain full-time staff working together in the development of standards for the World Wide Web.
If you experience any problems or get some errors in this check, the theme code have to edited by hand. If you're not familiar with liquid code, get in touch with a Shopify expert.
But have in mind that not every error is that bad. The named service will check your code against the current standard. Sometimes theme developer already use modern solutions that are working great with all browsers, but are not yet defined in the HTML standard.