Visualforce got an update!
One of the coolest updates within the Spring ’17 release notes I found was the ability to now natively use the Lightning Design System directly inside Visualforce without needing to upload or reference a static resource.
Why is this cool? Simple. You don’t contribute to the overall limit of how much space your static resources take up and you benefit from always having the latest version of the Lightning Design System incorporated into your pages.
In this release Salesforce has added the new apex:slds tag.