标签约束（Element Prohibitions）

下列标签定义了一些约束条件指明其可以包含哪些标签作为子元素（参见SGML Exclusions）。此约束适用于所有的层次的嵌套，即适用所有的后代元素。

a标签：不能包含其他a标签

pre标签：不能包含img、object、big、small、sub和sup标签

button标签：不能包含input、select、textarea、label、button、form、fieldset、iframe和isindex标签

label标签：不能包含其他label标签

form标签：不能包含其他form标签

此文系翻译之W3C中Element Prohibitions片段，原文如下：

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.