(sgml-basic-offset): New var.

(sgml-basic-offset): New var.
(sgml-name-re, sgml-attrs-re): New consts.
(sgml-tag-name-re, sgml-start-tag-regex, sgml-font-lock-keywords-1)
(sgml-mode): Use them.
(sgml-lexical-context): Default to (point-min) if nothing else works.
(sgml-calculate-indent): Indent slightly differently.
(sgml-indent-line): Use back-to-indentation.
(sgml-parse-dtd): New function.
(sgml-unclosed-tags): New var.
(html-mode): Set it.