I have created the simple NyanCat example project that comes with NME (purely as an example!) using the following line:
nme create NyanCat
I can compile (target) to Flash and it works, with the following line:
nme test project.nmml flash
But when I try the same for HTML5, with the following line:
nme test project.nmml html5
The files are created as expected, the browser appears, but a simple grey box is shown (see image). Firebug suggests the greybox in the image is a Canvas object with opacity set to '1'. There is also a canvas object above and below this one.
Checking through firebug everything is linked (the script files are there). I have tried Chrome and Firefox.
Try to setup local web server and open generated output from this server, not from filesystem.
On haxeflixel.com forum this subject was discussed lately. But haxeflixel now is down, so I can't post a link.
edit: Link to topic on haxeflixel.com - http://haxeflixel.com/forum/help/html5-test-page-not-showing-solved