通过使用HtmlNode.GetAttributeValue方法并将"margin-top" 魔术字符串设置为常数:

You could simplify your code a little bit by using HtmlNode.GetAttributeValue method, and making your "margin-top" magic string as constant:

const string margin = "margin-top: 0";
foreach (var pTagNode in pTagNodes)
{
    var styles = pTagNode.GetAttributeValue("style", null);
    var separator = (styles == null ? null : "; ");
    pTagNode.SetAttributeValue("style", styles + separator + margin);
}

不是很明显的改进，但是对于我来说，这段代码更简单.

Not a very significant improvement, but this code is simpler as for me.