Page min-height with dynamically added content not working

I've made a pag with a content div that has min-height set to 1200 pixels, and a black bacgkround color..
When I add a bunch of lines in the .aspx page (in html) , it works perfectly, it changes the black background height to the required height if it was more than 1200 pixels.
But, if I put a PlaceHolder in side the content div, and add a bunch of lines dynamically to the PlaceHolder, it doesn't work, the page shows the content with background of 1200 pixels of height, and at the bottom of it no more content background, but continues showing the rest of the lines outside of the content div..
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