DOCTYPE changes CSS behavior

It seems CSS behaves differently with and without the DOCTYPE comment. The HTML for my blog looks like

<!DOCTYPE html>
<html lang="">
<head>
        <meta charset="utf-8" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
...
</head>
...
</html>

And the page renders fine.

However, if I remove the !DOCTYPE comment from the header, something odd happens: the social network icons at the bottom of my page float up a little bit.

I really don't know what is happening.

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