Documents must be well-formed

Well-formedness is a new concept introduced by [XML]. Essentially this means that all elements must either have closing tags or be written in a special form (as described below), and that all the elements must nest.

Although overlapping is illegal in SGML, it was widely tolerated in existing browsers.

良好的xml文档结构，比如下面的代码不正确（overlapping elements）

here is an emphasized paragraph.

Element and attribute names must be in lower case

元素和属性名必须小写

For non-empty elements, end tags are required

标签必须有结束标志

Attribute values must always be quoted

属性值必须有引号

下面的 3 必须用引号括起来

Attribute Minimization

支持最少的属性。很多html的属性在 xhtml中消失了，比如 img align=”absmiddle”

Empty Elements

空元素也必须结束，如

Whitespace handling in attribute values

属性值中，头、尾的空格将会忽略，中间多个连续空格也将视为一个。

Script and Style elements

script 和 style 必须声明如下：

> & 等符号必须转义为 > & 等

另一种办法就是使用外部引用.js和.css文件。

SGML exclusions

SGML gives the writer of a DTD the ability to exclude specific elements from being contained within an element. Such prohibitions (called “exclusions”) are not possible in XML.

For example, the HTML 4 Strict DTD forbids the nesting of an ‘a’ element within another ‘a’ element to any descendant depth. It is not possible to spell out such prohibitions in XML. Even though these prohibitions cannot be defined in the DTD, certain elements should not be nested. A summary of such elements and the elements that should not be nested in them is found in the normative Appendix B.

The elements with ‘id’ and ‘name’ attributes

xhtml中只有a, applet, form, frame, iframe, img 和 map 标签具有name属性。其它只能用 id 。注意，将来所有name都会被放弃，全部使用id。

原文来自:

http://www.w3.org/TR/2000/REC-xhtml1-20000126/

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