What is SLDS used for?
The Salesforce Lightning Design System (SLDS) helps you build applications with the look and feel of Lightning Experience without writing a single line of CSS. SLDS is a CSS framework that gives you access to the icons, color palettes, and font that our developers use to create Lightning Experience.
How do I use SLDS in Salesforce?
To use Lightning Design System style sheets in your Visualforce page, add <apex:slds /> anywhere in your page markup. To use Lightning Design System style sheets in your Visualforce page: Add <apex:slds /> anywhere in your page markup. Set the <apex:page> applyBodyTag or applyHtmlTag attribute to false .