I created a CSS drop-down menu, all are fine.

parent-child sections are used in CSS, look like:
#parent > child{...}
#parent:hover > child{...}
Actually, the menu code was copied from somewhere then modified by me.
At beginning of the html page, I see:
<!doctype html public "-//w3c//dtd html 4.01//en" "http://www.w3.org/tr/html4/strict.dtd">
then, I reduce the code to:
<!doctype >
All are still fine.
But, menu doesn't work if without last <!doctype >.

I am wondering why the piece of code <!doctype > is so important? it sounds saying nothing.

Could you explain?