[jfoutz]

looks like the big trick is rendering to the background image:

    #mario-mushroom div {
      width: 170px;
      height: 140px;
      margin-left: -85px;
      margin-top: -90px;
      background-image: radial-gradient(circle at 50% 120%, rgba(0,0,0,0.7) 23%, rgba(0,0,0,0) 48%), linear-gradient(30deg, rgba(0,0,0,0.4) 10%, rgba(0,0,0,0) 20%), radial-gradient(circle at 50% 33%, #f8f6f7 32%, rgba(255,255,255,0) 32%), radial-gradient(circle at -13% 55%, #f8f6f7 20%, rgba(255,255,255,0) 20%), radial-gradient(circle at 113% 55%, #f8f6f7 20%, rgba(255,255,255,0) 20%), linear-gradient(to bottom, #ef0015 20%, #b2000c 100%);
      border-radius: 140px 140px 80px 80px;
    }

[jtheory]

Different gradients (sometimes linear, sometimes radial), and/or box-shadow rules like this:

  #camera div:after {
    width: 100px;
    height: 100px;
    margin-left: -20px;
    top: 15px;
    left: 50%;
    z-index: 3;
    background: linear-gradient(45deg, #ccc 40%, #ddd 100%);
    border-radius: 50%;
    box-shadow: 0 3px 2px #999, 1px -2px 0 white, -1px -3px 2px #555, 0 0 0 15px #c2c2c2, 0 -2px 0 15px white, -2px -5px 1px 17px #666, 0 10px 10px 15px rgba(0,0,0,0.3), -90px -51px 1px -43px #aaa, -90px -50px 1px -40px #888, -90px -51px 0 -34px #ccc, -90px -50px 0 -30px #aaa, -90px -48px 1px -28px rgba(0,0,0,0.2), -124px -73px 1px -48px #eee, -125px -72px 0 -46px #666, -85px -73px 1px -48px #eee, -86px -72px 0 -46px #666, 42px -82px 1px -48px #eee, 41px -81px 0 -46px #777, 67px -73px 1px -48px #eee, 66px -72px 0 -46px #666, -46px -86px 1px -45px #444, -44px -87px 0 -38px #333, -44px -86px 0 -37px #ccc, -44px -85px 0 -34px #999, 14px -89px 1px -48px #eee, 12px -84px 1px -48px #999, 23px -85px 0 -47px #444, 23px -87px 0 -46px #888;
  }

Neat results -- I'm not sure how practical these specific techniques are to everyday development, but of course a project like this provides the creator with a strong motivation to seriously master everything available in CSS; and the end result of that has obvious practical use.