At the introduction of CSS, two main browsers on the market, Microsoft’s Internet Explorer 4 and Netscape 4 did not fully support it. Netscape did horribly broke the entire standard, while IE came closer to it but with some important bugs. So, the developers had to create a different version of CSS for every browser, in order to be sure that their pages would render correctly on every one of them.
In time, when the standards became more important, the developers had to choose whether to create the next version of browser closer to the W3C
If you've had difficulties getting your web page to display correctly in more than one browser, you're not alone. The unlikely culprit might just be in the Doctype tag that you may or may not have added to your document.
QuirksMode goes into significant depth on the issue: "When Netscape 4 and Explorer 4 implemented CSS, their support did not match the W3C standard (or, indeed, each other). Netscape 4 had horribly broken support. Explorer 4 came far closer to the standard, but didn't implement it with complete correctness either. Although Explorer
HTML —which is short for HyperText Markup Language— is the official language of the World Wide Web and was first conceived in 1990. HTML is a product of SGML (Standard Generalized Markup Language) which is a complex, technical specification describing markup languages, especially those used in electronic document exchange, document management, and document publishing. HTML was originally created to allow those who were not specialized in SGML to publish and exchange scientific and other technical documents. HTML especially facilitated this exchange
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