Question

Is it semantic for a <figure> element to contain nothing but a <figcaption>? It does validate…

<figure>
  <img src="example.jpg">
  <figcaption>Image and caption</figcaption>
</figure>

<figure>
  <figcaption>Caption only</figcaption>
</figure>

In this context, it’d have ramifications for CSS styling:

figure figcaption { color: red; }

Answer 1

An img is not required in a figure element, it just happens to be common, and most examples that use figcaption also have an image.

http://dev.w3.org/html5/spec/single-page.html#the-figure-element

The <figure> element can thus be used to annotate illustrations, diagrams, photos, code listings, etc, that are referred to from the main content
of the document, but that could, without affecting the flow of the
document, be moved away from that primary content, e.g. to the side of
the page, to dedicated pages, or to an appendix.

http://dev.w3.org/html5/spec/single-page.html#the-figcaption-element

The <figcaption> element represents a caption or legend for the rest of
the contents of the figcaption element’s parent figure element, if
any.

Note the “if any” part, which leads me to believe that content other than the figcaption may not exist and it will still be valid. However, as Alochi notes, it suggests a misuse of the element since there is no content for the caption to describe.

There is an example in the current specification example of non-image content being accompanied by a figcaption, so I think it’s fair to assume it’s perfectly OK to not have an image:

Here, a part of a poem is marked up using figure.

<figure>
  <p>'Twas brillig, and the slithy toves<br>
  Did gyre and gimble in the wabe;<br>
  All mimsy were the borogoves,<br>
  And the mome raths outgrabe.</p>
  <figcaption>
    <cite>Jabberwocky</cite> (first verse).
    Lewis Carroll, 1832->  98
  </figcaption>
</figure>