Start and End Tags Must Match

Every start tag must have a corresponding end tag to properly delimit the content of the element the tags represent. The start and end tags are indicated exactly as they are in HTML, with "<" denoting the beginning of a start tag and "</" indicating the beginning of the end tag. The end delimiter of each tag is ">".

<ElementName>content</ElementName>

Empty Elements

An exception to the rule about start and end tags is the case in which an element has no content. Such empty elements convey information simply by their presence or possibly by their attributes, if any. Examples from XHTML 1.0 include:

<br />
<hr />
<img src="someImage.gif" width="100" height="200" alt="Some Image" />

An empty element begins like a start tag but terminates with the sequence "/>". Optional white space may be used before the two terminating characters. This author prefers to include a space to emphasize empty elements. The space before "/>" is necessary for XHTML 1.0 to be handled correctly by older browser versions. Of course, it's also possible to specify an empty element by using regular start and end tags, and this is syntactically identical (from the parser's viewpoint) to the use of empty-element notation.

<img src="someImage.gif" width="100" height="200" alt="Some Image"></img>

Note that just like in HTML (or more appropriately, XHTML), an empty element is often used as a separator, such as <br /> and <hr />, or to indicate by its presence a particular piece of data, or to convey metadata by its attributes. If the term empty element seems strange to you when attributes are involved, just think in terms of the content of the element. There is no content, even when there are attributes, which is why it's called empty.

Proper Nesting of Start and End Tags

No overlapping of start and end tags from different elements is permitted. Although this might seem like an obvious requirement, HTML as implemented by major browsers is considerably more forgiving and recovers for improper tag overlap. Correct nesting looks like this:

<OuterElement>
  <InnerElement>inner content</InnerElement>
</OuterElement>

An example of improper nesting is:

<OuterElement>
  <InnerElement>inner content</OuterElement>
</InnerElement>

Believe it or not, most browsers recover from this type of error in HTML, but they cannot and will not in XML or any language based on XML syntax. The improper nesting example results in either one or two fatal errors, with a message similar to this (depending on the parser):

Fatal error: end tag '</OuterElement>' does not match start tag. Expected
'</InnerElement>'
Fatal error: end tag '</InnerElement>' does not match start tag. Expected
'</OuterElement>'

Parent, Child, Ancestor, Descendant

The notion of the root element and the proper nesting rules leads us to some conclusions and terminology about the hierarchy of elements that are invariant across all XML documents. The terms ancestor and descendant are not used in the XML 1.0 Recommendation, but they certainly are in the DOM, XSLT, XPath, and so on.

• An element is a child of exactly one parent, which is the element that contains it.
• A parent may have more than one child.
• Immediate children and also children of a child are descendants of the parent.
• An element is an ancestor of all its descendants.
• The root is the ancestor of all elements.
• Every element is a descendant of the root.
• Every element has exactly one parent, except the root, which has no parent.

Attribute Values Must Be Quoted

In HTML (but not in XHTML), we are permitted to be inconsistent in the use of quotation marks to delimit the values of attributes. Generally, single-word values do not require quotes in HTML. For example, both of these are acceptable and equivalent in HTML:

<IMG SRC=someImage.gif>

<IMG SRC="someImage.gif">

In XML (and in XHTML), however, we are not allowed to be so cavalier about quotes. All attribute values must be quoted, even if there are no embedded spaces.

<img src="someImage.gif" />
<img src='someImage.gif' />
<img src="someImage.gif" width="34" height="17"/>

Notice that either single or double quotes may be used to delimit the attribute values. Of course, if the atribute value contains double quotes, then you must use single quotes as the delimiter, and vice versa.

<Book title="Tudor's Guide to Paris" />
<Object width='5.3"' height='7.1"' />

Elements and Attributes Are Case-Sensitive
XML Family of Specifications: A Practical Guide
White Space Is Significant