Client-side languages for web development include:
Usage: HTML / XHTML are the structural markup languages in use on the Web.
Versioning: The last version of HTML was 4.01, at which point development switched to XHTML.
Validation: The W3C offers a validator for HTML and XHTML.
Usage: CSS is the language used to control content and structure presentation on the Web.
Versioning: CSS is currently at version 2.1, although development continues on CSS3.
Validation: The W3C offers a validator for CSS.
Usage: JavaScript provides a level of interactivity not possible with HTML / XHTML and CSS alone.
Versioning: The most recent version of JavaScript is 1.5
Validation: There is no W3C validator for JavaScript, but browsers have debugging functionality to varying degrees.