Insert HTML from CSS
No. The only you can do is to add content (and not an element) using :before or :after pseudo-element.
More information: http://www.w3.org/TR/CSS2/generate.html#before-after-content
Content (for text and not html):
http://www.quirksmode.org/css/content.html
But just to be clear, it is bad practice. Its support throughout browsers is shaky, and it's generally not a good idea. But in cases where you really have to use it, there it is.
No you cannot. The only thing you can do is to insert content. Like so:
p:after {
content: "yo";
}