Because you don’t have to upload the Lightning Design System as a static resource, you can keep the syntax of your page simple and stay under the 250-MB static resource limit. To use Lightning Design System stylesheets in your Visualforce page, add <apex:slds /> anywhere in your page markup.
In general, the Lightning Design System is already scoped. However, if you set applyBodyTag or applyHtmlTag to false, you must include the scoping class slds-scope. Within the scoping class, your markup can reference Lightning Design System styles and assets.
To reference assets in the Lightning Design System, such as SVG icons and images, use the URLFOR() formula function and the $Asset global variable.