Identify a MAC over the Internet

I want to create a website which can read the MAC address of the system on which the user agent is running.

The reason I want to do this is, that I only want to display certain contents to a couple of devices. To be more specific: I know the MACs of two PCs, and whenever those PCs access my website (possibly from different networks/IP every time) I want them, and only them, to see a specific content.

I have no idea if this is the best approach? Is there a better way to do this? I would appreciate some advice.

My best, Christopher