I like this better than the PHP Markdown Extra syntax. Not only does it solve the look-ahead problem, but I always found placing the colon at the beginning of a line awkward (a notation that only Forth programmers can love … ).
Note that “definition lists” (I just can’t cope with the HTML5 renaming frenzy …) are not specific to HTML; similar elements exist
in DocBook as
in the NISO Journal Article Tag Set (JATS) as
<def-list>(properly named “definition list” …),
also in the ISO Standards Tag Set (ISOSTS) as
<def-list>too (since ISOSTS is an adaptation of NISO JATS), and
in ISO 12083:1994 there are similar
<dd>elements (again described as a “definition list” …) with this content model:
<!ELEMENT deflist - - ((head, ddhd)?, term, dd)>
So there is amply justification for having a dedicated and nice Markdown syntax for this kind of lists in my opinion.