CSS selectors
There are 4 types of selectors: simple (single HTML elements), 'in context' (multiple HTML elements), CLASS (a group of elements), or ID (a subgroup of elements).
Simple selectors
A single HTML element, such as H2, P, or B. For example:
H2 {color:green;}
will set the colour of every header 2 in the document to green. In this rule, 'H2' is the selector, and {color:green;} is the declaration.
To set all the headers:
H1,H2,h3,H4,H5,H6 {color:green;}.
Properties may be grouped for some properties, for example:
H2 {font: 10pt Courier italic;}
In context selectors
You can be more specific in defining which element you want to assign a style to. For example, to identify any italic tag contained within a paragraph tag, select P I.
P I {color:blue;}
CLASS selectors
The term CLASS identifies a grouping of elements. A period is written in front of the CLASS name.
CLASS .classname {color:red;}
ID selectors
The term ID identifies a smaller set within a CLASS, though it may sometimes be used interchangeably with CLASS. A hash mark or pound sign is always written in front of the ID name.
ID #IDname {color:green;}
Note: There should only be one tag in a document that specifies a particular ID.