Hidden features of HTML

Question

Using a protocol-independent absolute path:

<img src="//domain.com/img/logo.png"/>

If the browser is viewing an page in SSL through HTTPS, then it'll request that asset with the https protocol, otherwise it'll request it with HTTP.

This prevents that awful "This Page Contains Both Secure and Non-Secure Items" error message in IE, keeping all your asset requests within the same protocol.

Caveat: When used on a <link> or @import for a stylesheet, IE7 and IE8 download the file twice. All other uses, however, are just fine.

The label tag logically links the label with the form element using the "for" attribute. Most browsers turn this into a link which activates the related form element.

<label for="fiscalYear">Fiscal Year</label>
<input name="fiscalYear" type="text" id="fiscalYear"/>

The contentEditable property for (IE, Firefox, and Safari)

<table>
    <tr>
      <td><div contenteditable="true">This text can be edited<div></td>
      <td><div contenteditable="true">This text can be edited<div></td>
    </tr>
</table>

This will make the cells editable! Go ahead, try it if you don't believe me.

I think the optgroup tag is one feature that people don't use very often. Most people I speak to don't tend to realise that it exists.

Example:

<select>
  <optgroup label="Swedish Cars">
    <option value="volvo">Volvo</option>
    <option value="saab">Saab</option>
  </optgroup>
  <optgroup label="German Cars">
    <option value="mercedes">Mercedes</option>
    <option value="audi">Audi</option>
  </optgroup>
</select>

My favourite bit is the base tag, which is a life saver if you want to use routing or URL rewriting...

Let's say you are located at:

www.anypage.com/folder/subfolder/

The following is code and results for links from this page.

Regular Anchor:

<a href="test.html">Click here</a>

Leads to

www.anypage.com/folder/subfolder/test.html

Now if you add base tag

<base href="http://www.anypage.com/" />
<a href="test.html">Click here</a>

The anchor now leads to:

www.anypage.com/test.html

<img onerror="{javascript}" />

onerror is a JavaScript event that will be fired right before the little red cross (in IE) picture is shown.

You could use this to write a script that will replace the broken image with some valid alternative content, so that the user doesn't have to deal with the red cross issue.

On the first sight this can be seen as completely useless, because, wouldn't you know previously if the image was available in the first place? But, if you consider, there are perfect valid applications for this thing; For instance: suppose you are serving an image from a third-party resource that you don't control. Like our gravatar here in SO... it is served from http://www.gravatar.com/, a resource that the stackoverflow team doesn't control at all - although it is reliable. If http://www.gravatar.com/ goes down, stackoverflow could workaround this by using onerror.

The <kbd> element for marking up for keyboard input

Ctrl+Alt+Del