A valid HTML document has to include a document type declaration, which specifies which version of HTML is used in the document. There is usually no harm in leaving it out as browsers ignores this information, but other tools such as validators often require a document type declaration.
Alpha also uses the document type declaration to only let you use the HTML elements and attributes belonging to the HTML version specified. If a document contains a document type declaration then Alpha does the following.
Note. Alpha only understands the document type declarations for HTML 4.0 and ignores all other ones.
The specification for HTML 4.0 was released 18 December 1997 and is the current [April 98] official version of HTML. HTML 4.0 specifies three document type definitions (DTDs), which vary in the elements they support.
Strict
Includes all elements and attributes that have not
been deprecated (see below) or do not appear in
frameset documents. For documents that use this DTD, use this document type declaration:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
Transitional
Includes everything in the strict DTD plus deprecated
elements and attributes (most of which concern visual presentation such as
alignment and colors). For documents that use this
DTD, use this document type declaration:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
Frameset
Includes everything in the transitional DTD plus frames as
well. For documents that use this DTD, use this document type declaration:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Frameset//EN">
This document type declaration must be used and should only be used for documents defining the frames layout, i.e. documents containing FRAMESET and FRAME elements. For documents defining the content of the frames use either the strict or transitional DTD. If you want to use the attribute TARGET, then you have to use the transitional DTD.
Some elements and attributes are defined as deprecated in the HTML 4.0 specification. A deprecated element or attribute is one that has been outdated by newer constructs such as style sheets. Deprecated elements may become obsolete in future versions of HTML.
The deprecated elements are APPLET, BASEFONT, CENTER, DIR, FONT, ISINDEX, MENU, S, STRIKE, and U. The deprecated attributes are mostly the ones which concerns presentation, such as alignment and colors.
Alpha support some extensions to HTML 4.0 which were introduced by Netscape or Microsoft. For documents with a HTML 4.0 document type declaration these are obviously disabled. The elements which are extensions are MULTICOL, SPACER, NOBR, WBR, MARQUEE, BLINK, BGSOUND, EMBED, NOEMBED, KEYGEN, SERVER, LAYER, ILAYER, and NOLAYER.
In the attribute preferences dialog you can specify two preferences which are related to the DTDs.
Don't use extensions to HTML 4.0
HTML elements not included in HTML 4.0
will be disabled in the menu, and attributes not included in HTML 4.0 will
not be asked about. Exactly the same set of elements and attributes are
used as if the document had included a frameset document type declaration.
Don't use deprecated elements and
attributes
Deprecated elements will be disabled in the menu and
deprecated attributes will not be asked about (this includes the
extensions). The set of elements and attributes and those in the strict DTD
plus the ones used for frames. Use this setting when you are defining
frames documents but don't want to use deprecated elements or
attributes.
When any of these two settings apply and there is also a document type declaration in the document, the largest set of elements and attributes allowed both by the settings and the DTD is used.