• source code should be editable by line
• indentation is 4 spaces
• every child line of code must have one indentation level from its parent

http://www.python.org/dev/peps/pep-0008/

Example:

<div>
    <div>
        <p>
        It doesn't matter if this is indented
        </p>
    </div>
<div>

Example:

.foo,
.bar,
.test
{
    x: y;
}