Web Standards Validation
There are two primary aspects of a website that need validation:
You can use the W3C's QA Markup Validation Service.
The URL to test the main page of Philadelphia Reflections is http://validator.w3.org/
Firefox has several useful add-ons for (X)HTML validation; one that uses Tidy is here: Html Validator
The W3C has a validation service for CSS, too.
For Philadelphia Reflections, the following URL checks all the CSS definitions in the main page: http://jigsaw.w3.org/css-validator/ (note: this validator is a little flakey: it produces different answers for the same file; you have to refresh a couple of times to get the whole story)
Firefox has several useful web developer add-on tools; try this one: Web Developer
Once you've gotten the HTML and CSS basics under control, there are other aspects of your site that you will want to validate:
The W3C will check all your links for both response time and validity.
There is an absolutely lovely program called HTML Tidy, origianlly written by Dave Raggett and decribed by the W3C here: http://www.w3.org/People/Raggett/tidy/
Calls to Tidy are available in some newer renditions of PHP (sadly, not the one we are using), however, on Widows (only) versions of Firefox and Mozilla, you can download an extension that will provide all the Tidy functions in your browser! ... https://addons.mozilla.org/firefox/249/. This a fantastic feature that I use all the time.
Syndication XML Validation
Validating RSS and Atom files is greatly facilitated by http://feedvalidator.org/. It has a number of quirks, the worst of which is that it has a length limitation that we exceed and so we have to provide "short" syndication files since all the feed aggregators use this facility and reject any feeds that aren't validated by it.
Google Sitemap Validation
If you submit a sitemap to Google through their Webmaster Tools facility they will validate your sitemap when they load it. An external validation tool is available here: Validome Google Sitemap(s) Validator
Yahoo and Microsoft have agreed to support Google's Sitemap protocol and to support the inclusion of the line "Sitemap: http://www.philadelphia-reflections.com/sitemap.xml" in robots.txt. If other search engines adopt this facility it will make it much easier to get into the world's many search engines ... they'll pick up this line instead of us having to hunt them down.
Meta Tag Validator
As you puzzle the mysteries of search-engine indexing, you'll want to check your meta tags: http://www.widexl.com/remote/search-engines/metatag-analyzer.html
gzip Compression & Headers
When you start getting really fancy and want to include automatic gzip compression, you'll want to see it in action and you'll want to check out all of your HTTP headers: http://www.gidnetwork.com/tools/gzip-test.php
Of course, the reason you''re experimenting with gzip is because you're concerned about response time.
(1) Try this site for a detailed analysis: http://www.websitepulse.com/help/tools.php
(2) Firefox to the rescue again: FasterFox is another lovely add-in: https://addons.mozilla.org/firefox/1269/
Check the validity of your geo tags here: Geo-Tag Validator
http://uitest.com/en/analysis/ is the mother of all lists of validations routines