The Fragmention Syntax section states that an HTML ID cannot have whitespace, citing the HTML standard. However, a test case demonstrates that IDs with spaces are supported across browsers:

data:text/html,<div style="height: 10000px"></div><div id="space here">I am linked to</div>#space here

Whether or not this poses any practical problems remains to be seen, however the specification should at least explicitly acknowledge and state:

1. What browsers implement today, even if outside the HTML standard, and
2. How browsers would need to change that behavior in order for fragmentions to work as expected.