How to check your webpages code for errors

There are many automated online programs from checking web syntax

=Syntax=

HTML
checking HTML syntax
 * http://validator.w3.org/

CSS/Stylesheets
Validating CSS syntax, within HTML, or standalone CSS
 * http://jigsaw.w3.org/css-validator/

Javascript
Javascript code checking
 * http://www.jslint.com/

Java
Java code checker, These programs need to be run on your computer, and are not web-based
 * http://pmd.sourceforge.net/
 * http://checkstyle.sourceforge.net/
 * http://jalopy.sourceforge.net/

RSS/Atom feeds
Checking RSS feed's syntax
 * http://validator.w3.org/feed/

robots.txt
Checking syntax for robots.txt, using to control what pages search engines register
 * http://www.sxw.org.uk/computing/robots/check.html

=Optimizations=

Speed

 * http://www.websiteoptimization.com/services/analyze/

Broken links

 * http://validator.w3.org/checklink

Backwards compatability

 * http://www.delorie.com/web/wpbcv.html

mobileOK
Checking mobile
 * http://validator.w3.org/mobile/

=Other=

HTTP Status Codes Checker
Checks servers status codes, for File not found, script error, server error,...
 * http://www.seoconsultants.com/tools/headers.asp