So if you were able to combine the player.platform to see if they're on Windows, then check if HTML is enabled?
ie
NPC Code:
if (player.platform == "win") {
if (htmlenabled) {
sendpm <b>Welcome to the server!</b>;
} else {
sendpm Welcome to the server!;
}
} else {
sendpm Welcome to the server!;
}
Unless you can simplify it some more, which I'm sure is possible.