Why do web pages look different in different browsers?

HTML is an authoring language, it contains markup instructions (called tags) which instruct an interpreter (web browser) how to display parts of the text. At its simplest level a tag may instruct the browser to display some text in "heading level 1" style:

<h1>Heading 1 Style</h1>

However, quite how any browser will choose to interpret these tags is open to a degree of interpretation. Basically the heading 1 style is specified as the largest text size, but there are no hard rules about how large it should be, so while Internet Explorer may choose to display heading 1 style text at say 16 points, Netscape Navigator might choose to display heading 1 style text at 18 points high.

Further, browsers often allow the reader to change the behavior of the browser to suit their own preferences, select a different default font, change the relative size of displayed text and so on.

Realizing that HTML is not a hard specified publishing language, and therefore that it is open to interpretation by browsers, can save a novice from a lot of stress and worry about "why doesn't my web page look the same in IE5 and NN4?"

Matt