A Lancaster University Resource for Students and Staff
The World Wide Web Consortium (otherwise known as the W3C) is the organisation responsible for creating specifications to ensure that web pages comply to a standard. In simple terms, this means that many HTML tags have been "deprecated" and should no longer be used. HTML should be used to provide the page structure, but any styling of the page (e.g. text formatting, background colour, positioning) should be achieved using Cascading Style Sheets (CSS). See article by Eric Meyer for further information.
There is huge range of technology and web browsers in existence and yet there is no guarantee that these will display any given web page in a consistent fashion. By trying to persuade web developers to write to a standard it is hoped that future technological and software developments will result in products that can interpret all web pages exactly as intended by the designer.
You need to decide how high a standard you want to achieve (more about this under "Doctypes"). Then you need to make yourself aware of which HTML tags are legal and which have been deprecated. You can use on-line validators to check the code on your page and view an error report. More on this under "Validation".