Afaik div width property is almost useless if the parent set the display as flex, or if the div has no content.

That’s kinda the point of flex though. The width is determined by the flex rules; width 100% doesn’t make much sense in that context.

Not if the parent or ancestors don't have a width set... Hence why you need context.

The parent has _some_ width, even if it’s not explicitly set.

Yes, and I wish 100% would respect that implicit width but it doesn't. Try it

Any chance you could show an example? In theory a block level element can only sit inside another block level element. And those expand to the width of their parent unless you constrain the width.

Gotcha. That’s because of the 100% vs auto thing. These days everyone just puts “box-sizing: border-box” on everything to make it go away.

This explains a bit more about what you’re seeing

http://stackoverflow.com/questions/28353625/why-does-percent...

100% isn’t the default value, ‘auto’ is. Which has the effect of appearing 100% in many scenarios? But not all.

Fair, but actually that just means that the border and margin etc are pushed outside the container. In either case the div is operating by taking up the full width of the container.