I think Radiohead themselves would be the first to admit they weren't doing anything new. Rather, they took different "experimental styles" (some Aphex Twin / Autechre influence alongside some nods to Jazz and Krautrock) and molded it into something that was contemporary and accessible. The songs on Kid A and Amnesiac were still largely in the same verse-chorus-verse structure of say The Bends; they just changed the instrumentation around and had a darker and more introspective sound. It's "experimental" in comparison to what was happening in the mainstream sure, but beyond that it really isn't at all.
It depends on what your definition of "experimental" is, which is already a flawed term to flag around
as a genre because it doesn't have a solid definition. It's interpretations are many and there's
really no way to find solid ground because it's far more of an idea than a way to categorize music.
they are experimental. I don't think experimental necessarily equates to doing something "new" in terms of the music genre as a whole so much as it means stepping outside the band's musical comfort zone and incorporating new influences domestically
4 what it's worth i think Radiohead are more experimental (though less avant-garde) than some of the examples on this list but if some1's personal definition of experimental conflates it with avant stuff that's ok too