Placeholders
When working in prototype mode, the placeholder class is available to use. Depending on whether it is applied to an img tag, or something containing text, it will produce one of the below results.
Image
<img class="placeholder" width="400" height="300" />
Text
Some example text
Some longer example text
<p class="placeholder">Some example text</p>
<p class="placeholder">Some longer example text</p>