TBL based HTML on the emerging standard for text mark-up call the Standard Generalized Mark-up Language [SGML] which was being used at CERN and uses the now familiar mark-up tags contained with angle brackets "<tag>". An SGML document has two fundamental parts:
the document containing the content and the mark-up tags
a document type declaration (DTD) which defines the grammar and rules for the language in which the document is written. That is, it defines all the names of the various element use in the mark-up, how they interact with each other and other syntax that might apply.
Every SGML-based language, such as HTML, must have its own SGML DTD defining the language rules and, hence any SGML based document is useless without its associated DTD. SGML is good at what it was designed for (large scale document management systems) but its inability to process a document without a DTD, the inflexibility and inability of DTDs to be reused, plus the complexity of the software to process SGML documents meant that the authors of Web browsers never properly implemented the SGML interpreter and wrote software that could only process HTML codes.
Although processing of HTML pages by browsers was fast, this bespoke authoring for browsers meant that new features were difficult to implement consistently. Recognizing the special aspects of the Web a new sub-set of SGML was developed called the eXtensible Mark-up language [XML] which like SGML is a tool for defining other mark-up languages and the grammatical rules etc. are still defined by a DTD. XML imposes strict rules on how new languages must be written but, if followed, these rules make documents usable even if the DTD is not present. Some of these rules are:
Elements and attributes names are case-sensitive.
Every start tag must have an end tag.
Empty elements are written using "empty-element tags".
Element attributes must always be assigned values and these values must always be quoted.
Don't worry to much about the terminology at this stage - it will become clearer as this course develops.