iFrame border properties

[ankonaskiff17]

I have an iFrame to hold a weather radar image.
Default for iFrame is no border but I wanted one to act as border for radar image. Got border no problem and I defined color also but it behaved as if it had some sort of bevel type property that I don’t understand.
For example if I chose hex value for red, the bottom and right borders would be red, the top and left borders would be a darker red.
Anyone got any thoughts on what is going on there?