Markup Validator Web Service API
The w3c (X)HTML validator now has a webservices API
Anodyne » Blog Archive » Rspec custom matchers and be_valid_xhtml
custom RSpec matcher to validate XHTML using the w3 SOAP validation service
Validating RSS Feeds with RSpec
custom RSpec matcher to validate RSS feeds
lindsaar.net Tip #4 - Validating an Email Address with Ruby on Rails
Alex Kira's Blog: Rails Validation - using Validation Groups
some diuscussion on only validating some fields when saving a model
Jeff's Blog - Sanitizing POST params with custom Rack middleware
W3C’s Excessive DTD Traffic | W3C Systems Team’s Blog