Moving a site gradually form an asp + aspx mix pages web app to use new front end techniques like bootstramp, any advice?

Question

Iam working for a new client and It has a very old web app with some pages on asp (render as quirks mode IE 5.5) and some in aspx (render as IE7), the app only works on IE. Iam the only fonrt end developer in teh company and I would like to use new techniques like bootstramp and SASS but the site suses a framset with different frmaes inside.

Would you recommend me any adivce of how to start imprroving the web app?

Thanks, (Iam not sure where to start specially beacuse the frames are not allowing me to have a one page where to call evrything they are include libraries like Jquery on all the frames)

Any idea would help.. any advice

Thanks!