HTML

General formatting

• Use soft-tabs with a two space indent. Spaces are the only way to guarantee code renders the same in any person's environment.
• Paragraphs of text should always be placed in a <p> tag. Never use multiple <br> tags.
• Items in list form should always be in <ul>, <ol>, or <dl>. Never use a set of <div> or <p>.
• Every form input that has text attached should utilize a <label> tag. Especially radio or checkbox elements.
• Even though quotes around attributes is optional, always put quotes around attributes for readability.
• Avoid writing closing tag comments, like <!-- /.element -->. This just adds to page load time. Plus, most editors have indentation guides and open-close tag highlighting.
• Avoid trailing slashes in self-closing elements. For example, <br>, <hr>, <img>, and <input>.
• Don't set tabindex manually—rely on the browser to set the order.

<p class="line-note m-0" data-attribute="106">
  This is my paragraph of special text.
</p>

Boolean attributes

Many attributes don't require a value to be set, like checked, so don't set them.

<input type="checkbox" value="1" checked>

<select>
  <option value="1" selected>1</option>
</select>

For more information, read the WhatWG section.

Lean markup

Whenever possible, avoid superfluous parent elements when writing HTML. Many times this requires iteration and refactoring, but produces less HTML. For example:

<!-- Not so great -->
<span class="avatar">
  <img src="https://github.com/github.png">
</span>

<!-- Better -->
<img class="avatar" src="https://github.com/github.png">

Forms

• Lean towards radio or checkbox lists instead of select menus.
• Wrap radio and checkbox inputs and their text in <label>s. No need for for attributes here—the wrapping automatically associates the two.
• Form buttons should always include an explicit type. Use primary buttons for the type="submit" button and regular buttons for type="button".
• The primary form button must come first in the DOM, especially for forms with multiple submit buttons. The visual order should be preserved with float: right; on each button.

Tables

Make use of <thead>, <tfoot>, <tbody>, and <th> tags (and scope attribute) when appropriate. (Note: <tfoot> goes above <tbody> for speed reasons. You want the browser to load the footer before a table full of data.)

<table summary="This is a chart of invoices for 2011.">
  <thead>
    <tr>
      <th scope="col">Table header 1</th>
      <th scope="col">Table header 2</th>
    </tr>
  </thead>
  <tfoot>
    <tr>
      <td>Table footer 1</td>
      <td>Table footer 2</td>
    </tr>
  </tfoot>
  <tbody>
    <tr>
      <td>Table data 1</td>
      <td>Table data 2</td>
    </tr>
  </tbody>
</table>