CSS content property: is it possible to insert HTML instead of Text?

Unfortunately, this is not possible. Per the spec:

Generated content does not alter the document tree. In particular, it is not fed back to the document language processor (e.g., for reparsing).

In other words, for string values this means the value is always treated literally. It is never interpreted as markup, regardless of the document language in use.

As an example, using the given CSS with the following HTML:

<h1 class="header">Title</h1>

... will result in the following output:

<a href="#top">Back</a>Title

As almost noted in comments to @BoltClock's answer, in modern browsers, you can actually add some html markup to pseudo-elements using the (url()) in combination with svg's <foreignObject> element.

You can either specify an URL pointing to an actual svg file, or create it with a dataURI version (data:image/svg+xml; charset=utf8, + encodeURIComponent(yourSvgMarkup))

But note that it is mostly a hack and that there are a lot of limitations :

• You can not load any external resources from this markup (no CSS, no images, no media etc.).
• You can not execute script.
• Since this won't be part of the DOM, the only way to alter it, is to pass the markup as a dataURI, and edit this dataURI in document.styleSheets. for this part, DOMParser and XMLSerializer may help.
• While the same operation allows us to load url-encoded media in <img> tags, this won't work in pseudo-elements (at least as of today, I don't know if it is specified anywhere that it shouldn't, so it may be a not-yet implemented feature).

Now, a small demo of some html markup in a pseudo element :

/* 
**  original svg code :
*
*<svg width="200" height="60"
*     xmlns="http://www.w3.org/2000/svg">
*
* <foreignObject width="100%" height="100%" x="0" y="0">
*	<div xmlns="http://www.w3.org/1999/xhtml" style="color: blue">
*		I am <pre>HTML</pre>
*	</div>
* </foreignObject>
*</svg>
*
*/

#log::after {
  content: url('data:image/svg+xml;%20charset=utf8,%20%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20height%3D%2260%22%20width%3D%22200%22%3E%0A%0A%20%20%3CforeignObject%20y%3D%220%22%20x%3D%220%22%20height%3D%22100%25%22%20width%3D%22100%25%22%3E%0A%09%3Cdiv%20style%3D%22color%3A%20blue%22%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1999%2Fxhtml%22%3E%0A%09%09I%20am%20%3Cpre%3EHTML%3C%2Fpre%3E%0A%09%3C%2Fdiv%3E%0A%20%20%3C%2FforeignObject%3E%0A%3C%2Fsvg%3E');
}

<p id="log">hi</p>

In CSS3 paged media this is possible using position: running() and content: element().

Example from the CSS Generated Content for Paged Media Module draft:

@top-center {
  content: element(heading); 
}

.runner { 
  position: running(heading);
}

.runner can be any element and heading is an arbitrary name for the slot.

EDIT: to clarify, there is basically no browser support so this was mostly meant to be for future reference/in addition to the 'practical answers' given already.