So, I'm angry that I didn't know about this until now.
Check this out.
If you are using SASS or LESS, you can simply do this to make your grid:
.col-5 {
width: calc((100% / 12) * 5);
}
.col-7 {
width: calc((100% / 12) * 7);
}
That will create the proper calculation for your percentage based grid.
Also...did you realize there was a unit of measurement called "vw" that means "view-width"!?!
.css-12 {
width: 100vw;
}
See? "vw" doesn't give a damn about your containing div! "vw" will do what you TELL it to do! It's 100% the width of the view, man!
A really good example of what you can do with this, is break an image out of the container where it lives! (as seen here: calc() lets you do some real CSS magic)
img.bigImg { max-width:100vw; margin: 0 calc(-50vw + 50%); }
And even better? this is all just CSS. It's not LESS or SASS. It's just...CSS.