CSS Display defines Run-In Layout as

A run-in box behaves exactly as an inline-level box, except:

• If a run-in sequence is immediately followed by a block box that does not establish a new formatting context, it is inserted as direct children of the block box [...]
• Otherwise, an anonymous block box is generated around the run-in sequence and all immediately following inline-level content (up to, but not including, the next run-in sequence, if any).
• A run-in box with a flow inner display type inlinifies its contents.

The following example is provided:

<dl class='dict'>
  <dt>dictionary
  <dd>a book that lists the words of a language in alphabetical
      order and gives their meaning, or that gives the equivalent
      words in a different language.
  <dt>glossary
  <dd>an alphabetical list of terms or words found in or relating
      to a specific subject, text, or dialect, with explanations; a
      brief dictionary.
</dl>

.dict > dt {
  display: run-in;
}
.dict > dt::after {
  content: ": "
}

Which is formatted as:

dictionary: a book that lists the words of a language
in alphabetical order and explains their meaning.

glossary: an alphabetical list of terms or words found
in or relating to a specific subject, text, or dialect,
with explanations; a brief dictionary.

However, the dd establishes an inline formatting context according to CSS 2.1:

A block container box either contains only block-level boxes or establishes an inline formatting context and thus contains only inline-level boxes.

So I think that either the example is wrong and the dt should not be inserted inside the dd, or that the definition should allow a run-in sequence to be inserted as direct children of a block box which establishes an inline formatting context.