In (X)HTML there are 6 levels of headings, h1 through to h6. To maintain the logical structure of your documents, headings should follow a logical progression. Headings of level 3 shouldn't follow directly from headings of level 1, only from headings of level 2. You should always try to maintain a distinction between appearance and logic. Simply because an element is a heading of level 2 does not mean that it will be rendered larger and bolder than regular text in the document. Style sheets allow a web designer to very easily apply any styling to a heading. Keep in mind too, that there are browsers which don't display information in a visual way, relying on text to speech conversion, or braille conversion. For maximum accessibility, structural elements such as headings should be used correctly, and appropriately.
Hierarchy
Hierarchy is a top down organizational structure.
Horizontal Scrolling
Horizontal scrolling is scrolling sideways within a web browser's window. When a site has content that is wider than the browser window, users must scroll horizontally to see it. Horizontal scrolling should be avoided if possible. It's awkward to do and users hate it.
Hue
Hue is the name of a distinct color of the spectrum (e.g. red, green, yellow, orange, blue). It is the particular wavelength frequency.
HyperText Markup Language (HTML)
The markup language that is used on most of the World Wide Web to create web pages. The standards for HTML are controlled by the W3C.
