A CSS (cascading style sheet) allows to separate web sites content from its own style. As always use XHTML file to arrange the contents. One can format the contents and prepare a beautiful presentation with formatting of fonts, colors, background, borders, text so on…within a CSS file.
CSS is a major improvement in Web design world because it allows controlling the style and layout of multiple Web pages all at once. One can define a style for each HTML element and apply it to as many Web pages upon the requirement. To make a global change one need to simply change the style and all elements in the Web are updated automatically.
There are two ways to use the CSS, internal and external.
Internal Stylesheet means to simply add the CSS code within the tags of each HTML/XHTML page. With this method each and every HTML/XHTML file contains the CSS code needed to style the page. If any changes required then one will have to make the pages for all the pages. This method is good if one needs to style only one or two pages, or if you want different pages with different styles.
An external CSS file can be created with any text/HTML editor such as “Notepad” or “Dreamweaver”. One can save the file with the .css file extension. External style sheets enable to change the appearance and layout of all the pages just by editing one single CSS document. Hence, external Stylesheet can save lots of work and time too. One needs to link the file externally link in the head section of every HTML/XHTML file in which one need to use the Stylesheet:
From my point of view external Stylesheet is better because it is easier to maintain, reduce file size, improve flexibility and save lots of time.