HTMLSandBox is an online html testbed, powered entirely by Javascript. If you download this page and the linked files, it'll run right from your desktop!

All code below is XHTML 1.0 valid.

Doctypes
XHTML 1.0 Transitional <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
XHTML 1.0 Strict <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
XHTML 1.0 Frameset <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
Head Elements
Title <title>Title</title>
Content-Type <meta http-equiv="content-type" content="application/xhtml+xml; charset=UTF-8" />
Author <meta name="author" content="You" />
Description <meta name="description" content="About your site." />
Keywords <meta name="keywords" content="Your keywords." />
Robots <meta name="robots" content="noindex,noarchive,nofollow" />
Stylesheet <link rel="stylesheet" type="text/css" media="screen" href="URL" />
Shortcut Icon <link rel="shortcut icon" type="image/x-icon" href="URL" />
Javascript File <script type="text/javascript" src="URL"></script>
Headings
Heading <h1>Heading (1-6)</h1>
<h6>Heading (1-6)</h6>
Elements
Div <div>Div</div>
Paragraph <p>Paragraph</p>
Preformatted Text <pre>Text or Code</pre>
Blockquote <blockquote>Quote</blockquote>
Dividers
Break <br />
Horizontal Rule <hr />
Vertical Rule <hr style="height: 100px; width: 1px;" />
Text
Italic <em>Italic</em>
Bold <strong>Bold</strong>
Superscript <sup>Superscript</sup>
Subscript <sub>Subscript</sub>
Strikethrough <strike>Strikethrough</strike>
Abbreviation <abbr title="Abbreviation">Abbr</abbr>
Acronym <acronym title="Acronym">Acronym</acronym>
Tables
Table <table>
<tr>
<td>Content<td>
</tr>
</table>
Row <tr>
<td>Content<td>
</tr>
Heading <th>Content<th>
Data <td>Content<td>
Lists
Unordered List <ul>
<li>Item<li>
</ul>
Ordered List <ol>
<li>Item<li>
</ol>
Definition List <dl>
<dt>Definition Title<dt>
<dd>Definition</dd>
</dl>
Forms
Form <form method="post" action="result.php></form>
Input - Text: <input type="text" name="fieldName" value="Default Value" />
Input - Password: <input type="password" name="fieldName" value="Default Value" />
Input - Checkbox: <input type="checkbox" name="fieldName" checked="checked" />
Input - Radio: <input type="radio" name="fieldName" checked="checked" />
Input - Button: <input type="button" name="fieldName" value="Button" />
Input - Submit: <input type="submit" name="fieldName" value="Submit" />
Select: <select name="options">
<option value="1">Option 1</option>
<option value="2" selected="selected">Option 2</option>
<option value="3">Option 3</option>
</select>
Textarea: <textarea rows="2" cols="15">Textarea</textarea>
Media
Image <img src="URL" alt="Image description" />
Flash <object data="example.swf" type="application/x-shockwave-flash">
<div>If you can see this message, you do not have flash installed!</div>
</object>
Miscellaneous
Link (Anchor) <a href="URL">Link</a>
Helpful Links
HTML Validator W3C HTML Validator
HTML Tidy HTML Tidy (Fixes common HTML errors.)
EmEditor EmEditor (Our text editor of choice.)

All code below is CSS 2.0 valid.

Notes
Combining Styles You can combine styles. For example:
<span style="color: red; font-weight: bold; text-decoration: underline;">Bold, red, and underlined text.</span>
Measurement Units Millimeters: 10mm;
Centimeters: 10cm;
Inches: 10in;
Picas (1/6in): 10pc;
Points (1/72in): 10pt;
Pixels: 10px;
em-Space: 10em;
x-Space: 10ex;
Percentage: 10%;
Text
Italic <span style="font-style: italic;">Italic</span>
Bold <span style="font-weight: bold;">Bold</span>
Overline <span style="text-decoration: overline;">Overline</span>
Underline <span style="text-decoration: underline;">Underline</span>
Both <span style="text-decoration: overline underline;">Both</span>
Strikethrough <span style="text-decoration: line-through;">Strikethrough</span>
Color <span style="color: red;">Color</span>
Size <span style="font-size: 10pt;">Size</span>
Family <span style="font-family: Arial;">Family</span>
Horizontal Center
<p style="text-align: center;">Center</p>
Vertical Center
<p style="vertical-align: middle;">Center</p>
Text Indent
<p style="text-indent: 1em;">Indented Text</p>
Line Height <p style="line-height: 14px;">Line Height</p>
Element Styles
Padding <div style="padding: 10px;">Padding</div>
Margin <div style="margin: 10px;">Margin</div>
Border Width <div style="border-width: 2px;">Border Width</div>
Border Style <div style="border-style: solid;">Border Style</div>
Border Color <div style="border-color: #000;">Border Color</div>
Background Color <div style="background-color: #000;">Background Color</div>
Background Image <div style="background-image: url('URL');">Background Image</div>
Background Image Repeat <div style="background-repeat: repeat;">Repeat / Repeat-X / Repeat-Y</div>
Helpful Links
CSS Validator W3C CSS Validator
EmEditor EmEditor (Our text editor of choice.)
Just type in your code and press 'Post' - this box will auto-hide, and you can restore it by clicking «