In your proposal for CSS4, it was unclear whether you were referring to width on the x or y axis. Since this is a long horizontal line, it is most likely talking about the y-axis.
That said, It would be helpful to have a way to specify left and right margin/padding so that the underline can be extended beyond the text, or start halfway under the first letter.
In the future, it might be worth considering adding a lot of other things that can be done to a rectangle like shadows and such.
In your proposal for CSS4, it was unclear whether you were referring to width on the x or y axis. Since this is a long horizontal line, it is most likely talking about the y-axis.
That said, It would be helpful to have a way to specify left and right margin/padding so that the underline can be extended beyond the text, or start halfway under the first letter.
In the future, it might be worth considering adding a lot of other things that can be done to a rectangle like shadows and such.