@Override
public void handleMovePlayer(ClientboundPlayerPositionPacket packet) {
    // ...

    localPlayer.setPos(d1, d3, d5);
    localPlayer.setDeltaMovement(d0, d2, d4);
    float f = packet.getYRot();
    float f1 = packet.getXRot();

    if (packet.getRelativeArguments().contains(ClientboundPlayerPositionPacket.RelativeArgument.X_ROT)) {
        // If the player's current xRot is -90 and the "float f1" is -1, the player's xRot will be -91.
        // and the client sends -91 to the server.
        localPlayer.setXRot(localPlayer.getXRot() + f1);
        localPlayer.xRotO += f1;
        // fix example (this would not be a good way)
        // localPlayer.setXRot(Mth.clamp(localPlayer.getXRot() + f1, -90.0F, 90.0F));
        // localPlayer.xRotO = Mth.clamp(localPlayer.xRotO + f1, -90.0F, 90.0F);
    } else {
        // It's the same.
        localPlayer.setXRot(f1);
        localPlayer.xRotO = f1;
    }

    // ...

    this.connection.send(new ServerboundAcceptTeleportationPacket(packet.getId()));
    this.connection.send(new ServerboundMovePlayerPacket.PosRot(localPlayer.getX(), localPlayer.getY(), localPlayer.getZ(), localPlayer.getYRot(), localPlayer.getXRot(), false));
}