The six heading levels (H1-H6) indicate section headings. Heading
1(H1) is rendered as the largest and most important section heading while
Heading 6(H6) is rendered as the smallest (lowest importance) heading.
This attribute helps an author in situations with floating objects (images,
tables, etc.) produced through the Left and
Right ALIGN attributes. It allows content to stop being
flowed around the floated element.
[DEFAULT - No special line breaking
effects are applied relative to the floating element.] Left [breaks line after this element
and moves down vertically until the left margin is clear of floated objects.] Right [breaks line after this element
and moves down vertically until the right margin is clear of floated objects.] All [breaks line after this element
and moves down vertically until both margins are clear of floated objects.]
This is an SGML Document Access
(SDA) attribute. SDA attributes are designed to transform HTML (and
other SGML-based documents) to the ICADD
DTD - which is used in creating accessible documents for users with
visual disabilities (rendering in Braille, large print, speech
synthesis, etc.) The attribute values specify the corresponding name of
the element to convert the element to in the SDA element group (in this
case the 'H1'-'H6' heading elements - the mappings are the same in SDA
as they are in HTML.)
Historically, this element has commonly been used solely for its
font sizing features and even though the character formatting elements
BIG, FONT and SMALL are now available, headings can still be used for
this purpose (and Headings are still more widely implemented historically
than these other elements.) The recommended method currently for
specifying purely presentational instructions though is CSS.
Mixing Headings and FONT elements (especially those with SIZE attributes)
is possible, and is actually a good idea for backward compatibility.
The DTD for HTML 4 says that heading elements are not nestable. In practice
though, you'll find that authors have often historically done this,
and browsers have no problems rendering this sort of thing either.
Mosaic has always supported another heading size, H7, which has a lower importance
than H6 and is rendered smaller.
Netscape versions 1.0-1.22 only understood ALIGN values of LEFT and CENTER.
Support for ALIGN=RIGHT began in version 2.0.
The 'justify' value of the ALIGN attribute is currently only supported in
the 4.0 Beta 2 and above versions of Netscape and Internet Explorer.