Handheld devices such as smartphones, tablets, and PDAs are commonly used for online shopping. In order to satisfy web page requests on these devices, web developers are using a responsive web design approach, where websites dynamically adapt to different screen sizes and orientations. One of the challenges faced by web developers is to provide a usable experience across different devices. Motivated by this, a mobile-first usability guideline was established by extracting good usability practices from five popular responsive e-commerce websites. Validation of the usability guideline was conducted, and results indicate that it will have a positive impact on the overall user experience.