[sedatk]

Microsoft Edge already does that. They show a quite prominent "Not secure" sign with an exclamation mark instead of the regular hollowed out (aka very indistinguishable) lock icon when the connection isn't trusted HTTPS.

[madeofpalk]

All browsers to do this now, to varying levels of severity.

Firefox gets a padlock with a red slash through it, Chrome gets that warning icon with "Not secure", and Safari just says "Not secure".