The W3C standard includes specifications for HTML, XML, XHTML and CSS, for example. The respective W3C standard is initiated, developed and maintained by the World Wide Web Consortium (W3C for short), whose founder and chairman is Tim Berners-Lee. Berners-Lee also created one of the first W3C standards himself with the first versions of HTML.
The W3C standards are merely recommendations and in no way binding standards. Membership of the W3C is open to any type of organization. Web developers in particular are familiar with the content of the W3C standards.
Further references:
https://www.loginradius.com/engineering/need-validate-site-w3c/
The following video provides a brief introduction to the W3C:
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