Your web pages will have a chance of rendering properly on everyone's browser if you validate your HTML. Internet Explorer (the majority of your traffic) accepts invalid or HTML and still shows your page just fine. Netscape, Mozilla, Firefox/Firebird, Opera, and Safari really require valid HTML for proper operation. Your page will display incorrectly, and it may be unreadable.
»validator.w3.org/ is the standard HTML validator. You can give it HTML or XHTML source or have it read your pages directly from your web site. It's an invaluable tool.
When you click on the Valid HTML link the validator will run on the referer, in this case your page.
If you see lots of errors on your page, try fixing them one at a time, starting with the first one reported. Most of the time a syntax error one place will look like two (or more) errors.
You must have a DOCTYPE as the first non blank line on each page in order to have valid HTML/XHTML. If you are just starting out, and you aren't sure what DOCTYPE to use, try using HTML 4.01 Transitional, the DOCTYPE sure to be standards compliant, yet allowing for the widest interpretation of the rules.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Once your pages are valid HTML/XHTML, they have a great chance of being seen by everyone visiting your site.
A CSS Validator examines your Cascading Style Sheet and checks for syntax errors. It does not tell you when you have done something that doesn't make senst to the browser. The current specification for CSS is CSS 2.0.
If you are troubleshooting a problem with the CSS code, it's always a good first troubleshooting step to revalidate the style sheet.
You can place a link to this service on your site. This allows you to validate your CSS with one click! Just use »jigsaw.w3.org/css-validator/validator and add ?uri=your-css-source. Like this: CSS 2.0.
Using a CSS Validation Service is only half the story. You should also validate your HTML and XHTML by using a Markup Validation Service. Use both to ensure your site is coded properly and will be presented well to all user agents.
There are a number of free editors that people use. In the end, it boils down to personal preference as to which is "best".
Please check the licensing for each piece of software. While some of what is listed is wide-open, some is free for personal use only.
•Notepad: basic text editing •Notepad ++: A souped-up version of Notepad, with code coloring, collapsing, and more. •NVU: a free WYSIWYG editor, in the genre of Dreamweaver. Cross platform (Win, Lin, Mac), fairly robust, standards-aware. •HTML-KIT:"HTML-Kit is a full-featured editor designed to help HTML, XHTML and XML authors to edit, format, lookup help, validate, preview and publish web pages."> •XStandard: "XStandard is the leading standards-compliant plug-in WYSIWYG editor for Windows desktop applications and browser-based content management systems (IE/Mozilla/Firefox/Netscape)"