HTML (current version 4.0) is standardized and set by the World Wide Web consortium (W3C). Their role is to define the standard versions of HTML. However not unsurprisingly it is a language that continually changes and grows. Many software companies create browsers with extra tags (so called extension tags) that are not part of the current HTML standard.
Although this is important in the development of HTML language it can cause problems of incompatibilities between browsers so be wary of using them
an example is <bgsound> which IE supports. This defines a audio file which plays as background without any need of plug ins or downloading audiofiles. Many other browser do not support this.
Although we've said throughout that HTML is case insensitive, ignores spaces (unless the one between word), Tabs and carriage returns sadly not all browsers do. So watch out for extra spaces. These can radically alter the appearance of your page when viewed on different browsers
Remember to close your tags e.g. if using italics don't forget </i> when finished. If your document doesn't work properly often this is the source of your problems
Oh!!! never use The <blink> tag. Try it once only on a page and you'll get the idea how annoying it is to Netscape users!!!!