Helvetica Neue in action on dev.twitter.com

Under the hood

.header-1 { font-family: "Helvetica Neue LT", "Hiragino Kaku Gothic Pro", Helvetica, Tahoma, Arial, sans-serif; font-size: 48px; font-style: normal; font-weight: 600; letter-spacing: normal; line-height: 52px; text-transform: none; color: #24292C; }
.header-2 { font-family: "Helvetica Neue LT", "Hiragino Kaku Gothic Pro", Helvetica, Tahoma, Arial, sans-serif; font-size: 40px; font-style: normal; font-weight: 600; letter-spacing: normal; line-height: 44px; text-transform: none; color: #23527C; }
.content-1 { font-family: "Helvetica Neue LT", "Hiragino Kaku Gothic Pro", Helvetica, Tahoma, Arial, sans-serif; font-size: 16px; font-style: normal; font-weight: 300; letter-spacing: normal; line-height: 24px; text-transform: none; color: #24292C; }
.content-2 { font-family: "Helvetica Neue LT", "Hiragino Kaku Gothic Pro", Helvetica, Tahoma, Arial, sans-serif; font-size: 16px; font-style: normal; font-weight: 300; letter-spacing: normal; line-height: 24px; text-transform: none; color: #24292C; }