When running a w3.org check on the HTML on the who is online page, there is a colour parse error in the legend.
The issue is on groups that don't have a colour assigned to them like the 'newly registered users' group.
Who Is Online Page Legend Error Snippet:
The unusual thing is, there is a legend on the index page too which uses the same HTML code as the one on the viewonline page but that does not have this issue as you can see below.
Index Page Legend Snippet:
To replicate this error, you will need to go into the 'newly registered users' group settings in the ACP and tick the 'display on the legend' option.
(ACP/Users And Groups/Manage Groups/Display group in legend)