A lot of websites we're making nowadays are HTML/CSS with a touch of javascript out of necessity since nobody wants to just use some wordpress template or have it look like every other AngularJS webapp. It might just be the particular sort of sites I stumble upon or my strict adblocking though.
But I'm not sure it could ever be fully 'mainstream' since there's enough people (particularly older) that need big buttons and familiar design to browse the web. But for smaller sites that cater to generally younger people, it could take this direction. Only time will tell!
PS: I agree that it really isn't brutalist, but there really should be a better word to describe this sort of aesthetic.
... I began by replacing the image carousels with pictures of William Howard Taft, America's greatest president by volume.
It's even more important for web design. Give me simple HTML with a touch of css, and javascript only if it's absolutely necessary. I can think of hardly any websites that I would consider "beautiful" these days for exactly this reason.
http://idlewords.com/talks/website_obesity.htm
http://www.webdirections.org/blog/the-website-obesity-crisis...
PS: I'm not sure I would classify these sites as brutalist; perhaps 'utilitarian' or 'functional' would be better descriptors.