How to capture the HTML tags generated by asp.net code?

Hello all,
 
My requirement is to catch the event of an HTML tag being generated because of an asp.net server side control.  I want to add an attribute in the HTML tag. For e.g. the following asp.net code
<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeFile="Default.aspx.cs" Inherits="_Default" %>
<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <h2>
        Welcome to ASP.NET!
    </h2>
    <p>
        To learn more about A ...