For anyone who develops web pages (or is considering it), it is important to make sure they are compliant with web standards so they will appear correctly in all browsers. Check your HTML source against the W3C (the organization responsible for developing web standards) validator:

HTML/XHTML Validator: [http://validator.w3.org/]
CSS Validator: [http://jigsaw.w3.org/css-validator/]

If there are any inconsistencies or structural errors, you will be notified, as well as suggestions on how to fix them.