Preceding sibling selectors.

Like these but the other way around: tag + tag and tag ~ tag

I think no more globals is all we need. Or make ShadowDOM the only valid way of styling.

<select> element = freedom style

As you posted I too want to design Check Boxes and in addition to that I need an easier way to apply "vertical-align: middle" property. For Example current situation is like this

.parent{
  display:table;
}

.child{
  display:table-cell;
  vertical-align:middle;
}

I want something like this

.child{
  vertical-align:middle;
}

What i would like to see is what you already mentioned. We are working on a View Layer for a Windows Application and want to throw away our old "windows xp" look and set a new layer on top of the application with the web.

Some things are really a pain to change with pure css like checkboxes, radiobuttons and scrollbars.

I know you should normally NOT change the look of the scrollbars, but we have a really dark and minimalistic design, and the bright 3D scrollbars of some browsers just don't match.

Improvement to Flex Boxes

Flex Boxes haven't solved all the problems. When Flex direction is row and we have divs with variable heights, there are gaps created in between multiple rows.

Less Code -

.grid-container{ //Wrapper's Class
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
}

.grid{  //Grey boxes' Class
  background: #aaa;
  height: 100px;
  width: 200px;
  margin: 10px;

  &:nth-child(1){
    height: 200px;
  }
  &:nth-child(2){
    height: 300px;
  }
  &:nth-child(3){
    height: 100px;
  }
  &:nth-child(4){
    height: 250px;
  }
  &:nth-child(5){
    height: 200px;
  }
  &:nth-child(6){
    height: 300px;
  }
}

Codepen Link — codepen.io/anon/pen/mJXdxP

Ancestors Selection

I would say "Ancestors Selection" is the most needed feature in CSS4. It is very difficult to select elements from a descendent and travel up the tree in HTML. Many developers have asked for this feature in past. Let's see what they come up with in CSS4.

Something like following will be very helpful:

.header < .logo{
	...
}

or

.logo:parent{
	...
}