This needs a little bit of tweaking on a per-site basis, but works really well at blocking most things.

  */ Hide most everything that isn't the content. /*
  :not(.main-content) { 
    display: none !important; 
  } 
  .main-content > * { 
    display: block !important;
  } 

I can read content more or less marketing/fluff free. I then add several styles to make things a bit more legible (font size, a specific font I prefer, body width 85%, line height, and less-contrasting colors).

When I have some spare time on a future weekend I plan to find a way to combine all of them into a single Firefox add-on or a one-click-to-do-everything bookmarklet.