https://addons.opera.com/sr-me/extensions/details/w3c-markup-validation-service/
W3C Markup Validation Service ekstenzija - Opera dodaci
This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc.
w3c markup validation serviceoperadodaci
https://validator.w3.org/
The W3C Markup Validation Service
W3C's easy-to-use markup validation service, based on SGML and XML parsers.
markup validationw3cservice