Is it possible to display an .html document , or .html fragment at CSS content?

The CSS content property allows you to specify content to be inserted into an element. This content can be text, an image, or a generated value.

Can you display an image at the CSS content property of an HTML element at a url() value of content?

Yes, you can display an image at the CSS content property of an HTML element using a url() value of content.

For example:

.content {
  content: url(image.png);
}

This will display the image image.png as the content of the .content element.

Can you display an .html document or .html fragment at the CSS content property of an HTML element?

No, you cannot display an .html document or .html fragment at the CSS content property of an HTML element. This is because the content property only supports content that can be rendered as a single element, such as text or an image.

See also CSS Generated Content Module Level 3

The CSS Generated Content Module Level 3 defines the content property and its various use cases.

What happens when no formats are supported, and the author does not explicitly indicate a fallback?

If no formats are supported and the author does not explicitly indicate a fallback, the content property will be empty.

Why doesn’t an element fallback to its contents unless an author explicitly says so?

An element does not fallback to its contents unless an author explicitly says so because the CSS content property is intended to be used to insert additional content into an element, not to replace the element's existing content.