I'd say they were (Blairite) New Labour which is slightly left than the Lib Dems. The newer Corbynite Labour supporters don't like the Guardian because the newspaper has negative coverage of their figurehead from time to time.

The thing with newspapers is that they keep posting opinion pieces using the same page design/layout as any other editorial or news piece with a small heading that says "Opinion", thus making it seem serious, and at the same time when they post opinion pieces that go way over the line they use the "but it was just an opinion piece" card. They want to have their cake and eat it as well.

I think that's a problem of media literacy and not page templating.

Sections and categories are not new features for newspapers or magazines. Words have meaning—they're not there for no reason.

I think a good solution would be to have opinion pieces in another subdomain such as blogs.mynewspaper.com/ephraimgoldstein with a very different design from the rest of the paper.

Maybe. I'm saying more that there really isn't a problem on the paper's end.

The problem, if there exists one at all, is the reader ignoring words on the page of a publication that is expressly about transmitting words.

That said, the Guardian already uses a completely different highlight colour for its Opinion section than it uses for its News section.

Like the way editorials in print newspapers are always left justified (ragged right), while news articles are fully justified[0]. It's subtle, but for anyone who's been reading newspapers regularly, it's immediately obvious.

[0] At least, that's the case here in Canada.

On the web site, the opinion pages (including capital-E Editorials) have a cream background, while everything else has a white background.