lightningStylesheets, apex:slds and apex:includeLightning in Salesforce.

lightningStylesheets=”true”

Make Visualforce apps look better in Lightning.

<apex:slds/>

To use markup specified by SLDS in Visualforce page.

There are three steps to add Aura components to a Visualforce page.

1. Add the Lightning Components for Visualforce JavaScript library to your Visualforce page using the <apex:includeLightning/> component.

2. Create and reference a Lightning app that declares your component dependencies.

3. Write a JavaScript function that creates the component on the page using $Lightning.createComponent().