rolls need to not appear to come from the player they should be contained in system message otherwise the player can just copy the html from a roll and manually tweek it to whatever and use that as his next roll.
css should be something like
#textchat .content > .rollresult {
margin: 5px;
border: 2px solid #FDD;
border-radius: 8px;
}
#textchat .formula:before,
#textchat .rolled:before {
content:"FAKE";
color:red;
}
#textchat .content > .rollresult > .formula:before,
#textchat .content > .rollresult > .rolled:before {
content:"";
color:transparent;
}
That will make it a little harder then just C/P the roll HTML to fake a roll
