Double underline tag?

Use a border and and underline:

.doubleUnderline {
    text-decoration:underline;
    border-bottom: 1px solid #000;
}

<span class="doubleUnderline">Test</span>

Here's a working fiddle.

You can try to add this:

h1.dblUnderlined { border-bottom: 3px double; }

Note: The width must be 3px or greater because it represents the total width; not the width of each border. As you increase the measurement, the width of the lines and space may or may not be equal based on the divisibility of the defined measurement by 3. A remainder of 1, and 1 is added to the width of the space; a remainder of 2 will result in 1 being added to each line.