sgml-tag-alist

sgml-tag-alist is a variable defined in `sgml-mode.el'.
Its value is

(("!["
  ("ignore" t)
  ("include" t))
 ("!attlist")
 ("!doctype")
 ("!element")
 ("!entity"))

• This variable may be risky if used as a file-local variable.

Documentation:
Alist of tag names for completing read and insertion rules.
This alist is made up as

(("tag" . TAGRULE)
...)

TAGRULE is a list of optionally t (no endtag) or `\n' (separate endtag by
newlines) or a skeleton with nil, t or `\n' in place of the interactor
followed by an ATTRIBUTERULE (for an always present attribute) or an
attribute alist.

The attribute alist is made up as

(("attribute" . ATTRIBUTERULE)
...)

ATTRIBUTERULE is a list of optionally t (no value when no input) followed by
an optional alist of possible values.

You can customize this variable.