XHTML标签禁令

指出了，那些标签内部不能再包含哪些标签。来自http://www.w3.org/TR/xhtml1/

Element Prohibitions

This appendix is normative.

The following elements have prohibitions on which elements they can contain (see SGML Exclusions). This prohibition applies to all depths of nesting, i.e. it contains all the descendant elements.

a

must not contain other a elements.

pre

must not contain the img, object, big, small, sub, or sup elements.

button

must not contain the input, select, textarea, label, button, form, fieldset, iframe or isindex elements.

label

must not contain other label elements.

form

must not contain other form elements.