About css rules
A CSS formatting rule consists of two parts—the selector and the declaration (or in most cases, a block of declarations). The selector is a term (such as p, h1,a class name, or an id) that identifies the formatted element, and the declaration block defines what the style properties are. In the following example, h1 is the selector, and everything that falls between the braces ({}) is the declaration block:
The style (which comes from a rule, or a collection of rules) resides in a place separate from the actual text it’s formatting—usually in an external style sheet, or in the head portion of an HTML document. Thus, one rule for h1 tags can apply to many tags at once (and in the case of external style sheets, the rule can apply to many tags at once on many different pages). In this way, CSS provides extremely easy update capability. When you update a CSS rule in one place, the formatting of all the elements that use the defined style are automatically updated to the new style.
h1 {
font-size: 16 pixels;
font-family: Helvetica;
font-weight:bold;
}
An individual declaration consists of two parts, the property (such as font-family) and value (such as Helvetica). In the previous CSS rule, a particular style has been created for h1 tags: the text for all h1 tags linked to this style will be 16 pixels in size, Helvetica font, and bold.The style (which comes from a rule, or a collection of rules) resides in a place separate from the actual text it’s formatting—usually in an external style sheet, or in the head portion of an HTML document. Thus, one rule for h1 tags can apply to many tags at once (and in the case of external style sheets, the rule can apply to many tags at once on many different pages). In this way, CSS provides extremely easy update capability. When you update a CSS rule in one place, the formatting of all the elements that use the defined style are automatically updated to the new style.
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