Okay, firstly, your briefing name is called "Briefing" - I suggest it be lowercase, okay?
Secondly, when I export a mission, then I export the same one, I don't think it overwrites for me. So you might wanna ALT + TAB and delete the .pbo you exported, then export.
Plus, I think the best way to remm/comment something out would be "<!-- TEXT HERE -->" - not "<! TEXT HERE >". BIS use "<! --- TEXT HERE>".
Plus, in HTML, you can't have something like this:
Hints:
-Stay in first person mode, it makes it easier to judge the terrain
-If you crash your bike will explode
-You have no boost on this one
It won't work like that mate. It has to be like this:
Hints:
<br>
-Stay in first person mode, it makes it easier to judge the terrain
<br>
-If you crash your bike will explode
<br>
-You have no boost on this one
"<br>" means the next line - "<br><br>" means two lines. Also, you used the paragraph code in as well ("<p>TEXT HERE</p>") - I'm not sure if OFP supports that (<p> is a better alternative than <br>/<br><br> but I'm not sure if it'll work still).
I attached a briefing.htm file from one of the STOCK missions. Use that. Just edit it to your needs. But don't play around with the real complicated stuff - just the text and basic code like <br>, etc.
Hope that helps.
Chris