HTML defines a series of six heading levels whose associated numbers are intended to signify the relative importance of the section below the heading. The H1 element represents the most important, whereas H6 represents the least important. HTML document parsers could examine a page's tags to create a table of contents based on the headings. This means that for proper document structure, these heading levels should be used in proper sequence, without skipping levels for aesthetic purposes.

It is up to the browsers to determine the font, weight, and other characteristics of each level. Each heading element is rendered on its own line, with no line break or paragraph elements necessary to begin the content of the section titled with the heading. By and large, this pattern applies to other browser versions and operating systems except for Navigator on the Macintosh, whose default H4 and H6 elements render characters wider (albeit shorter) than the H3 and H5 elements preceding them.

You can always override the browser's rendering style for any heading level or individual heading with style sheet rules.

Determines how the heading is rendered in physical relation to the next outermost container, usually the body. Both Navigator and Internet Explorer support alignment values for center, left, and right alignment. HTML 4.0 adds the possibility of a fully justified alignment, as well.

The ALIGN attribute is deprecated in HTML 4.0 in favor of the text-align: style sheet attribute.