HTTP conditional requests - HTTP
validators all conditional headers try to check if the resource stored on the server matches a specific version.
...such values are called validators, and are of two kinds: the date of last modification of the document, the last-modified date.
... together with the resource, the validators are sent in the headers.
...And 2 more matches

Validator - MDN Web Docs Glossary: Definitions of Web-related terms
validators can be created for any format or language, but in our context we speak of tools that check html, css, and xml.
... learn more general knowledge validator on wikipedia short list of validators ...

Priority Content - Archive of obsolete content
keller equivalents security developer central -> security plugins central -> plugins devedge community -> mozilla web developer community notes from devmo wishlist devedge toolbox tools on devedge examples on devedge started: devedge sidebar completed: standards-compliant authoring tools on devedge mostly completed: tools:validators on devedge dependant on the sidebar tabs and the tune-up wizard.

Gecko Compatibility Handbook - Archive of obsolete content
the toolbox aggregates several invaluable w3c tools for diagnosing web page problems as well as validators, code samples, and more.

Standards-Compliant Authoring Tools - Archive of obsolete content
related links validators html central ...

Fixing Incorrectly Sized List Item Markers - Archive of obsolete content
in situations where validation is a priority, segregate the rule into a separate stylesheet so that the rule will not confuse css validators run against the main stylesheet.

Index - MDN Web Docs Glossary: Definitions of Web-related terms
validators can be created for any format or language, but in our context we speak of tools that check html, css, and xml.

Getting started with Ember - Learn web development
for more information on the technical aspects of the glimmer vm, the github repository has some documentation — specifically, references and validators may be of interest.

Links and Resources
online accessibility validators there are now a few available and excellent online accessibility validators which check single webpages according to checkpoints of wcag guidelines and then reports errors or warnings.

Index - Firefox Developer Tools
143 validators tools, website validator, html5 validator, validator this document lists different resources for developers to validate web pages.

Window.open() - Web APIs
protocol scheme "javascript:" will be reported as an error by link validators and link checkers.

ETag - HTTP
strong validators are ideal for comparisons but can be very difficult to generate efficiently.