Problem migrating HTML to ASPX

Hey!
Sorry if this is the wrong forum, but It's a simple question..
Designer handed me website in HTML and im converting it to an ASPX site to be able to use the server controls.. I know its just dragging the markup in the asp file, however there is a CSS file that styles all the <form> tags, so when I get to the main <form runat="server"> it screwes up the site, is there a way to avoid that specific tag of getting affected by CSS ?

Thanks for the help!
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