Why use definition lists (DL,DD,DT) tags for HTML forms instead of tables?

I guess it's up to you to determine the semantics, but in my opinion:

Rather than a definition list, form-related properties should be used.

<form>
  <label for="fullname">Full Name:</label>
  <input type="text" name="fullname" id="fullname">
  <label for="email">Email Address:</label>
  <input type="text" name="email" id="email">
</form>

The "for" attribute in the <label> tag should reference the "id" attribute of a <input> tag. Note that when labels are associated with fields, clicking the label will put the associated field in focus.

You can also use tags like <fieldset> to cluster sections of a form together and <legend> to caption a fieldset.

I've successfully used the technique outlined in this article several times.

I agree with sjstrutt that you should use form related tags like label and form in you forms, but the HTML outlined in his example, will often lack some code you can use as "hooks" for styling your form with CSS.

As a consequence of this I markup my forms like this:

<form name="LoginForm" action="thispage">
    <fieldset>
        <legend>Form header</legend>
        <ul>
            <li>
                <label for="UserName">Username: </label>
                <input id="UserName" name="UserName" type="text" />
            </li>
            <li>
                <label for="Password">Password: </label>
                <input id="Password" name="Password" type="text" />
            </li>
        </ul>
    </fieldset>
    <fieldset class="buttons">
        <input class="submit" type="submit" value="Login" />
    </fieldset>
</form>

This approach leaves me with a comprehensible set of tags, which contains enough hooks to style the forms in a lot of different ways.

This is a subset of the issue of semantics vs formatting. A definition list says what they are, a list of related key/value attributes, but does not say how to display it. A table says more about layout and how to display the data then what the data inside is. It limits how the list can be formatted both by overspecifying the format and by underspecifying what it is.

HTML, historically, has mixed up semantics with formatting. Font tags and tables being the worst examples. The move to CSS for the formatting and the stripping of a lot of the pure formatting tags out of XHTML restores, somewhat, the separation of meaning from formatting. By separating formatting into CSS you can display the same HTML in many different ways reformatting it for a wide browser, a small mobile browser, printing, plain text, etc...

For enlightenment, visit the CSS Zen Garden.

In this case, labels and inputs are your semantic meaning and they stand on their own.

Imagine you had to read the web page, out load, to a blind person. You wouldn't say "Okay, I have a list of definitions here. The first term is 'name'." Instead, you'd probably say "Okay we have a form here and it looks like the there's a set of fields, the first input is labeled 'name'."

This is why the semantic web is important. It allows the content of the page to represent itself accurately to both humans and technology. For example, there are many browser plugins that help people quickly fill out web forms with their standard information (name, phone number, etc). These rarely work well if inputs don't have associated labels.

Hope that helps.

Definition lists have semantic meaning. They are for listing terms (<dt>) and their associated definitions (<dd>). Therefore in this case a <dl> portrays the semantic meaning of the content more accurately than a table.

Definition lists are almost never used because semantically speaking they rarely show up on the internet.

In your case the correct code has been posted:

<form>
    <label for="fullname">Full Name:</label>
    <input type="text" name="fullname" id="fullname">
    <label for="email">Email Address:</label>
    <input type="text" name="email" id="email">
</form>

You are creating a form with inputs and labels for said inputs, you are not setting forth a list of words and defining them.

If you are doing some kind of help section then definition lists would be appropriate, e.g.:

<dl>
    <dt>HTML</dt>
    <dd>Hypertext Markup Language</dd>
    <dt>CSS</dt>
    <dd>Cascade Stylesheets</dd>
    <dt>PHP</dt>
    <dd>Hypertext Preprocessor</dd>
</dl>

Part of the reason for using <dl> for marking up the form is that it is much easier to do fancy CSS layout tricks with a <dl> than a <table>. The other part is that it better reflects the semantics of the form (a list of label/field pairs) than a table would.

Ok, table hate is part of it too.

Sometimes, a definition list simply presents the information in the way that's desired, whilst a table does not. Personally, I would probably not use a definition list for a form, unless it suits the style of the site.

Virtually all forms are tabular. Do you ever see a form that's not tabular? The guidelines I've read suggested using the table tag for tabular presentation and that's exactly what forms, calendars, and spreadsheets are for. And now they're using DD and DT instead of tables? What is the Web coming to?! :)