You can turn off CSS in many browsers. It is under the Accessiblity dialog in IE, at least in 6.x. Firefox also supports disabling CSS in older versions, and so does Opera before the crippling WebKit migration.
I can't imagine it being that difficult to implement as a feature. If the setting is on, you would skip interpreting any tags and style= attributes.
Past versions of many browsers support selecting between multiple stylesheets specified by the page, as well as provide your own stylesheet to use in place or in addition to the page's stylesheet.
(This was one of the main, amazing fucking benefits of CSS, to separate presentation from content, so that users who were hindered by the page's styles could turn it off and still be able to view the content.
And all of it works great if you do it right, because the styles cascade, and, I apologize for repeating myself, the content layer is separate from the presentation layer.
At some point all these features went away, 'cause fuck accessibility, I guess.
Meanwhile, CSS became Turing complete, so security-wise it's like having a second JavaScript which cannot be turned off.
Great progress all around. /s
When people ask me why I spend time on ensuring that I support older browsers in my projects, this is one of the reasons. Sometimes I just want to use a decent fucking browser that's not been lobotomized, and at least I can do it on my own sites.