Whitney in action on twilio.com

• .header-1 { font-family: "Whitney SSm A", "Whitney SSm B", "Helvetica Neue", Helvetica, Arial, sans-serif; font-size: 11px; font-style: normal; font-weight: normal; letter-spacing: 1px; line-height: 19px; text-transform: uppercase; color: #FFFFFF; }
• .header-2 { font-family: "Whitney SSm A", "Whitney SSm B", "Helvetica Neue", Helvetica, Arial, sans-serif; font-size: 50px; font-style: normal; font-weight: 200; letter-spacing: normal; line-height: 55px; text-transform: none; color: #FFFFFF; }
• .header-3 { font-family: "Whitney SSm A", "Whitney SSm B", "Helvetica Neue", Helvetica, Arial, sans-serif; font-size: 24px; font-style: normal; font-weight: 300; letter-spacing: normal; line-height: 30px; text-transform: none; color: #FFFFFF; }
• .content-1 { font-family: "Whitney SSm A", "Whitney SSm B", "Helvetica Neue", Helvetica, Arial, sans-serif; font-size: 18px; font-style: normal; font-weight: 300; letter-spacing: normal; line-height: 32px; text-transform: none; color: #FFFFFF; }

Whitney is available from MyFonts.