Here's a few links to resources on other websites. They're links to authoritative information (I won't link to anything that is misleading, nor cannot be authenticated).
World Wide Web Consortium (W3C), the people who set the HTML specifications. Anybody designing HTML rendering agents (e.g. web browsers), who knows what they're doing, will be designing it to adhere to these specifications. Writing HTML to the ratified specifications ensures the widest possible audience for your website. The site also has other WWW authoring related information (e.g. making websites widely accessible).
Instructional information on authoring websites, and avoiding problems. This is a more tutorial based site, compared to the two sites above, which could be regarded more as reference material.
A campaign to promote developing websites that work in any webbrowser. The the any browser website is a promotional attempt at the campaign, the morons page is a blunt object to strike at annoying webmasters with.
The latest version of the Hyper-Text Mark-up Language (HTML), the language that web pages are written in. The latest version now being "Extensible HTML" (XHTML), but requires you to also read the HTML 4 docs to see a manual of the HTML elements and attributes, or read the very terse lexicon of XHTML elements. Though, HTML 4 is still your best bet for compatibility with the largest number of browsers.
The latest version of HTML 4.
ISO HTML (the only officially specified version of HTML, though the W3C is the recognised authority figure).
Cascading Style Sheets (CSS) specifications. Used to add style control (presentation and layout) to websites, to pretty them up; but in a manner that isn't required for the site to be of use to people. Non-CSS capable browsers will ignore the CSS statements, and the page will render in a plainer fashion.
(Links to the ECMAScript language [third edition] and components specifications, compact profile [third edition], and XML (E4X) specification, respectively.)
Sun invented Java, Microsoft created a bastardised incompatible version of it. Sun's Java engine is used in various different browsers, even the later versions of IE can use it; authoring for Sun's Java, ensures wider compatibility.
An on-line validator, to check web pages for HTML errors.
An on-line validator, to check web pages for HTML errors. They also offer a free downloadable validator, that you can run with your own webserver.
An on-line validator, to check for CSS validity.
Useful information, but not authorative, not validated, etc.
This site has a longish list of different browsers that are available. Unfortunately, it's little more than a list of different browsers, with links to pages where you could download the browsers (i.e. didn't have details about each browser, that I could see; it's not even categorised into browsers for different computer operating systems). The links were also a series of pages to click through, eventually ending up at a notice about where they could be downloaded from (providing direct links to the homepages of each browser, rather than to archive sites, may have been more practical; as I've done below, for Opera and Mozilla).
Some information about different browsers, lists of them, and other information.
Opera is one of the most standards compliant, and secure, web browsers. It's also a reasonably small sized file to download. It's written to adhere to the HTML, CSS, and ECMA script specifications, rather than trying to go their own way. If a website works in Opera, then it should work for just about all other browsers. It's available for a wide variety of operating systems, and a very handy browser for checking your pages, as you can very simply, and individually, toggle on and off; CSS interpreting, image loading, plug-ins, proxying, cookies, etc. (That's not referring to “error checking”, by the way. No browser is suitable for that task, as browsers typically try to work their way around errors, rather than protest about them. But Opera does give you a quick and simple way to see how pages will look with, and without, images, styling, or scripting, etc.)
Mozilla is now an open-source project (i.e. the program code is not a secret, anybody can see exactly how it works, and use the information). There's a Mozilla webbrowser, and other browsers use its technology as their core (e.g. Beonex, K-meleon, and recent versions of Netscape). It's programmed with a goal of being “standards compliant”, rather than hotch potch (as per MSIE), and is under peer review, rather than being at the mercy of a corporation only interested in making money. Mozilla is also available for various different computer operating systems, and competent programmers could create versions for even more systems.