Some tags have attributes. An attribute is a name,
followed by an = sign, followed by the value. For example, to make
a centered H1 heading, use the ALIGN
attribute with value center; i.e.
<h1 align="center">A level one heading</h1>
Attribute names are also case-insensitive. The double quotes around the value are optional unless the value contains embedded white space.
For more information about HTML see Larry Aronson and Joseph Lowry's The HTML 3.2 Manual of Style (Ziff-Davis Press, 1997) or the NCSA Beginner's Guide to HTML.