CSS is a relatively recent development in web design that expands upon HTML by allowing greater control of text inside HTML documents.
For example, by using CSS instead of traditional HTML tags, it is possible to separate content from its modifying code, making possible the use of external CSS documents to control the formatting and layout of entire multipage web sites. The advantages of this innovation become immediately apparent when one considers the 'old' html standard, where every change required complex opening and closing tags.
For example, let's say that we wanted to define several lines of text in a web page as having these attributes: Typeface: Ariel, Helvetica; Bold;, Italic. This would, in the 'old' days, require that each instance be enclosed in tags that included all of these html instructions.
With CSS, we can define these attributes in a single CSS file that is separate from the HTML file. Instead of defining all the attributes of the style change inside the HTML document itself, we do this in the CSS file, naming the style something like 'h1', and then apply the opening and closing tags 'h1' to the text inside the htm document.
This allows for greater ease in keeping multpage sites consistent in terms of design. If a text style needs changing, it can be done by simply modifying a single CSS file, instead of opening and editing multiple pages. This is sometimes referred to as 'Global' editing (controlling multiple instances of a style change from a single location).