<asp:label id="lbl" runat="server" font-size="10px" text="some text" xmlns:asp="#unknown"></asp:label>
in javascript :
var jquery = $('#lbl').css('font-size');
// (or)
var javascript = document.getElementById('lbl').style.fontSize;
c#:
var siZe = lbl.Font.Size.Unit.ToString();
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if you html tag is like this
<asp:Label ID="lbl" runat="server" Style="font-size: 15px" Text="some text"></asp:Label>
you can use:
var df = lbl.Style["font-size"];