The BYU ECEn Dept. XML Web Templates: XML Fundamentals

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General XML Fundamentals

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The six things you must know about XML... Every bit of information in an xml document is wrapped in tags. Each piece of information must be wrapped in a starting and ending xml tag. There must be a single outer-most tag. There are two characters you can only type with special codes: '&' and '<'. XML is case-sensitive. There is more than one way to create an empty tag.

General XML Fundamentals is a short list explaining several important aspects of XML, with special emphasis when XML differs from HTML.

The six things you must know about XML...

1 Every bit of information in an xml document is wrapped in tags.

Every bit of information in an xml document is wrapped in tags. Tags describe content within xml files. Check out these examples:

<tag> Information within the tag </tag>

<text> This part of the content is described as text </text>

<related-page> This part of the content is described as a related page </related-page>

2 Each piece of information must be wrapped in a starting and ending xml tag.

Each piece of information must be wrapped in a starting and ending xml tag. XML will complain about this:

<text> Oh no! No ending tag!

If you know HTML, be careful: XML differs from HTML on this point. Where HTML handles missing closing tags perfectly in many cases and expects them, XML will always complain when a tag does not open and close.

3 There must be a single outer-most tag.

There must be a single outer-most tag. You can't have this:

<tag> Arghhh! Two outer-most tags! </tag> <tag> Arghhh! Two outer-most tags! </tag>

Instead you need something like this:

<outer-tag> <tag> Hurrah! The 'outer-tag' saves the day! </tag> <tag> Hurrah! The 'outer-tag' saves the day! </tag> </outer-tag>

Note: In the ECEn XML Web Templates, the <page> ... </page> tag will usually be your outermost tag and wrap around everything.

4 There are two characters you can only type with special codes: '&' and '<'.

There are two characters you can only type with special codes: '&' and '<'. If you want an '&' or a '<' to show up on your page, you need to type '&' (without the single quotes; think of 'ampersand') or '<' (without the single quotes; think of 'less than'), respectively.

5 XML is case-sensitive.

XML is case-sensitive. For example, this is valid XML:

<TEXT>This is my text.</TEXT>

But, the ECEn Web Templates will not render the content of <TEXT> tags as they would the content of <text> tags, because in XML, the two are different tags.
Understanding that XML treats <TEXT> and <text> explains why XML would complain about something like this:

<TEXT>This is my text.</text>

Here XML sees an opening tag with no closing tag and a closing tag with no opening tag, which since everything in XML must be enclosed in an opening and closing tag, is not valid XML.

If you know HTML, you probably know that HTML is not case-sensitive.

6 There is more than one way to create an empty tag.

Tags do not need to contain any information to be valid. This is valid XML:

<line-break></line-break>

XML has a short hand way to represent a single empty tag:

<line-break/>

These two representations are equivalent, so feel free to use whichever you prefer when you insert an empty tag.