How do I know that I have built a technically good website and not a hard to maintain mess that doesn’t work on all browsers?

There are a number of  World Wide Web Consortium (W3C) tools I use regularly to check a website adherence to  the grammar, vocabulary and syntax of the technology used.  At the WSC validation site they address why Web professionals choose to validate. They are:

1. Validation is useful as a debugging tool
2. Validation is useful as a future-proof quality check
3. Validation eases maintenance
4. Validation helps teach good practices
5. Validation is a sign of professionalism

Web developer tools I use a lot:

1. HTML Checker: Checks Validity of HTML, XHTML, SMIL, MathML, etc use to code a site:  http://validator.w3.org
2. CSS Checker: Checks Cascading Style Sheets (CSS) and (X)HTML documents with style sheets:  http://jigsaw.w3.org/css-validator/  Note:  If you want to validate aCSS style sheet embedded in an (X)HTML document, you should first check that the (X)HTML you use is valid.
3. Link Checker: Check links and anchors in Web pages or full Web sites: http://validator.w3.org/checklink  Note: This Link Checker looks for issues in links, anchors and referenced objects in a Web page, CSS style sheet, or recursively on a whole Web site. For best results, it is recommended to first ensure that the documents checked use Valid (X)HTML Markup and CSS.
4. Mobile Checker: Checks if your Web site is mobile-friendly:  http://validator.w3.org/mobile/
5. Unicorn:  Performs as many of the above checks as possible:  http://validator.w3.org/unicorn/  Note:  the output is useful if you are familiar with the above checkers.

Web developer tools I use only in special situations:

1. Feed Validation Service: Checks the syntax of news feeds in formats like Atom or RSS: http://validator.w3.org/feed/
2. Semantic Extractor – Sees a Web page from a semantic point of view. Extracts such information as outline, description, languages used, etc: http://www.w3.org/2003/12/semantic-extractor.html
3. RDF Validator – Checks and visualizes RDF documents: https://validator.w3.org
4. P3P Validator – Checks whether a site is P3P enabled and controls protocol and syntax of Policy-Reference-File and Policy: http://www.w3.org/P3P/validator

Remember: These are tools not physical laws.

A perfect score is not always worth the effort and is even possible.  For example, I use WordPress for this site and that results in a number of syntax errors generated from WordPress engine.   So to me its a trade off.  I could do this site in pure HTML/CSS, but I’m saving time using WordPress.  If this was a commercial site I would take the effort to fix the last hand full of errors.

Unfortunately,  building a technically correct website does not not mean you have a good website.  But that is another discussion.