HTML

Type .html in the first line of an input cell and start writing raw HTML on a new line:

.html
Here is my
<h3>Hello World</h3>

Here is my
<h3>Hello World</h3>

The input is also processed with PHP-like template engine (WSP) allowing to embed Wolfram expressions:

.html
<h3>Today is <?wsp Now // TextString ?></h3>