Internet Explorer 6

Were I to go back, I would keep it simple.

There would be a button at the top that said, “upgrade”. You didn't need to click it.

The page loaded instantaneously. It was simple black text on a white background. There was an image or a few, tiny. Just floated to let the content wrap and flow around it.

If you clicked upgrade, the image version of the webpage will be loaded. Your browser width would be sent and a screenshot would be taken in the swankiest available rendering engine, or loaded from cache, and downloaded. The page structure would be used to add clickable hotspot regions to the screenshot.

That's it.