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Leftmargin, topmargin, etc were never including in the HTML recommendations of the W3C for any version. As hakre mentions, there are CSS solutions and those are what is recommended.
A site search here on "leftmargin" or "topmargin" will turn up many helpful threads. In short, to get rid of the gaps at the side of your design, you probably want a CSS rule something like this:
body {
margin: 0;
padding: 0;
}