XHTML

What Is XHTML?

If you run an older website with XHTML, there’s no negative SEO effect — Google renders both formats the same way. Switching to HTML5 is still worthwhile because it is more future-proof and offers better tools for structured data and modern web features. If you’re planning a relaunch anyway, replace XHTML directly with HTML5.

XHTML is a stricter, XML-based variant of HTML that was developed in the 2000s to make web standards more precise. While HTML is relatively tolerant and accepts even faulty syntax, XHTML requires exact adherence to rules — all tags must be closed, attributes must be in quotation marks, and the structure must be valid XML. XHTML was an attempt to standardize HTML but was ultimately replaced by HTML5, which is equally strict but better suited to modern browsers.

Technically, XHTML is served with the MIME type “application/xhtml+xml,” which modern browsers today barely support — many older browsers fall back to HTML rendering. This makes XHTML irrelevant for current SEO projects. Some legacy websites still use it if they haven’t been updated in a long time. From an SEO perspective, it doesn’t matter whether XHTML or HTML is used — Google renders both the same way.

In practice, XHTML is historical knowledge today. New websites should use HTML5, which is more modern, flexible, and optimized for all browsers. If an old XHTML website needs SEO optimization, a relaunch to HTML5 should be considered — not because of SEO penalties, but because HTML5 is technically more future-proof and offers better tools for structured data.