At the top of each of your pages, you need a document declaration. Yes, it must.
If you do not specify a document type, your HTML is not a legitimate HTML, and most browsers will use "quirks mode" to process the page, which means that the browser thinks you don't know what you are doing and will handle your code the browser's own way. You can be an HTML master who is invincible on the earth, or your HTML can be flawless and CSS can be perfect, but if there is no document declaration or a wrong document declaration, your web page is exactly the same as a short-sighted, one-eyed gibbon baby.
The documentation declaration for XHTML 1.0 Strict is as follows:
Sample code [www.CuoXIn.com]