HTML Tidy
Console tool and library that corrects and cleans up HTML and XML
HTML5test
Online test of browser support for HTML5 features
httest
Script-based tool for testing and benchmarking HTTP applications
http_load
Multiprocessing HTTP test client for measuring web server throughput
HTTP::Recorder
Perl proxy that records web interactions as WWW::Mechanize scripts
httperf
Tool for measuring web server performance