Put concisely, so nobody can inject content in-transit.

Use an ISP that you can trust.

For most of us in the US, that isn't an option. Also, your ISP isn't the only point of attack for injections.

Search engines and browsers are quickly headed towards essentially requiring HTTPS. So probably want it from that stand point.

And, first thing I can think of is affiliate links. The site is all affiliate links, but I can just MITM it and just sub in my own number and get credit.