You can ignore the rules described on this page if your web page is a simple one HTML page that does not use external CSS resources. But the more sophisticated your web application is, then the more reliable your CSS architecture shoudl be.
Do not use inline statements.
The inline statements can not be generic. There is no easy way to overwrite their values, and the usual way to change their values is changing the source itself.
Exclusion: sometimes the inline statements are desirable if they are the source for a dynamically changing values with Java Script. Example:
<div dojoType="dijit.layout.ContentPane" title="Tab 2" style="display:none">
If you did not move your inline/embedded css statements from the source file today, there is no any guaranty that you will do it in the future.
Do not use !important
unless there is no better way to overwrite some existing css statement.
Include external css in the next order
<link type="text/css" rel="stylesheet" href="main.css"> (generic statements)
<link type="text/css" rel="stylesheet" href="container.css"> (specific statements)
<link type="text/css" rel="stylesheet" href="dialog.css"> (more specific statements)