Gmail is updating to provide responsive design for all devices. For emails that support it, the Gmail app will scale the content of the email to fit different screen sizes. This means text that’s easier to read, bigger buttons and links, and wider images. You’ll also see these changes on the desktop as well.
If you’re a developer, you can achieve this by using CSS media queries to make sure your email is formatted correctly. You can change styles based on width, rotation and resolution.
Example:
@media screen and (min-width: 500px) {
.colored {
color:red;
}
}
You can find the full list of supported CSS rules in the developer documentation.