Markup Tags.

[I am not an SGML expert, so I may be using the wrong terminology or be making other errors in the following. Later I'll check it against an SGML reference book, and update the terminology. Meanwhile I'd appreciate being told about any errors or ambiguities.]

HTML-Math defines various SGML markup tags for specialized purposes.

All of these tags are valid only within the SGML MATH context. [?]

The HTML-Math tags are of just two types, in terms of their SGML syntax definitions:

empty, i.e. the tag stands alone as an element (according to SGML), and looks like
```
  <tagname>
```
(note that the HTML-Math parser, which is independent of the SGML parser, may treat this empty element as a term, or as an infix or prefix or postfix operator, or etc, according to the current Math Syntax Model; see "Syntax" and "Extensibility")
begin/end, i.e. the tags must always come in matched pairs, i.e.
```
  <tagname> ... </tagname>
```
(in this case, the HTML-Math parser will treat these tags as grouping operators, so they will always be correctly matched)
Some of these tags have named attributes; these are either specific to HTML-Math and are discussed along with the individual tag, or are general to all HTML tags.
The begin/end tags all have the same content model, which allows any sequence of PCDATA (in the MATH context) and HTML-Math elements (and some to-be-discussed subset of HTML elements). Any possible content (which passes the DTD) is legal, though in some cases nonstandard content may have no effect on rendered output.
[There are specific markup tags given as proposals or as examples in the documents "Syntax" and "Macro processing", but none of these have been discussed by the group. There will also be specific markup tags discussed in "Rendering and layout primitives" and perhaps in "High-level constructs".
For convenience, here is the list of tags given as proposals or examples in the other documents at the time of writing: Macro processing:
```
<mscope> ... </mscope>          delimits scope of macro definitions

<mdef> ... <as> ... </mdef>     represents macro rule
                                [the SGML syntax might not require the <as>]

<mp>                            precedes named formal parameters in macro rules


Syntax (fully expanded form, for author-optional SGML compatibility):

<mn> ... </mn>                  designates number literal (in expanded form)
<mi> ... </mi>                  designates identifier
<mo> ... </mo>                  designates operator
<me> ... </me>                  designates non-atomic expression
]
```
(end of "Markup Tags")