Documents as trees

DOM stands for Document Object Model. This is the specification of how a HTML and XML documents has to be structured, as well as how the file is manipulated to create, edit or remove contents.

We can think of DOM as a tree structure:

<?xml version="1.0" encoding="UTF-8"?>
<TextCorpus lang="de">
    <text>Karin fliegt nach New York. Sie will dort Urlaub machen.</text>
    <tokens>
        <token ID="t_0">Karin</token>
        <token ID="t_1">fliegt</token>
        <token ID="t_2">nach</token>
        <token ID="t_3">New</token>
        <token ID="t_4">York</token>
        <token ID="t_5">.</token>
        <token ID="t_6">Sie</token>
        <token ID="t_7">will</token>
        <token ID="t_8">dort</token>
        <token ID="t_9">Urlaub</token>
        <token ID="t_10">machen</token>
        <token ID="t_11">.</token>
    </tokens>
</TextCorpus>

XML

XML stands for EXtensible Markup Language. This language was designed to store and transport data. And it was designed to be both human- and machine-readable. Unlike HTML the structure of the document, the elements, their attributes, and the content are not pre-defined. That provides a very flexible framework.

XML is a generalized way of describing hierarchical structured data. An XML document contains one or more elements, which are delimited by start and end tags.

<s>This is a sentence.</s>

Elements can be nested to any depth. An element inside another one is said to be a subelement or child. The first element in every XML document is called the root element. An XML document can only have one root element.

<s>
    <token>This</token>
    <token>is</token>
    <token>a</token>
    <token>sentence</token>
    <token>.</token>
</s>

Elements can have attributes, which are name-value pairs. Attributes are listed within the start tag of an element and separated by whitespace. Attribute names can not be repeated within an element. Attribute values must be quoted. You may use either single or double quotes.

<s id="s_0">
    <token pos1="DT" pos2="DET">This</token>
    <token pos1="VBZ" pos2="VERB">is</token>
    <token pos1='DT' pos2='DET'>a</token>
    <token pos1='NN' pos2='NOUN'>sentence</token>
    <token pos1='.' pos2='PUNCT'>.</token>
</s>

If an element has more than one attribute, the ordering of the attributes is not significant. Element’s attributes form an unordered set of keys and values. There is no limit to the number of attributes you can define on each element.

<s id="s_0">
    <token pos1="DT" pos2="DET">This</token>
    <token pos2="VERB" pos1="VBZ">is</token>
    <token pos1="DT" pos2="DET">a</token>
    <token pos2="NOUN" pos1="NN">sentence</token>
    <token pos1="." pos2="PUNCT">.</token>
</s>

Elements can have text content. Elements that contain no text and no children are empty. Elements that contain text and children elements are said to contain mixed content.

This is an element with text content:

<s>This is a sentence.</s>

This is an empty element:

<comment type="gesture"/>

This is an element with mixed content:

<s>This is a sentence with <italics>mixed</italics> content.</s>

Finally, XML documents can contain character encoding information on the first line, before the root element.

<?xml version="1.0" encoding="UTF-8"?>
<s>
    <token>This</token>
    <token>is</token>
    <token>a</token>
    <token>sentence</token>
    <token>.</token>
</s>

(Mark Pilgrim. Dive Into Python 3. http://www.diveintopython3.net/xml.html)

Well-formed and valid

Web browsers are quite lenient regarding not well-formed and invalid HTML. They will try to figure out how to render a page, even if there are errors. However, errors in XML documents will stop your XML applications. XML parsers will choke, XML errors are not allowed.

Therefore, whenever you work with markup languages, try to check that everything is alright to be sure that your material is error free. Follow this piece of advice and you will avoid lot of headache in the future.