BEM Notation with Double Dashes Is Being Deprecated
Why: Double dashes are not supported in XML files that include comments. This notation change was first announced in Salesforce Lightning Design System (SLDS) Release 2.3.1 on May 25, 2017.
How: In Summer ’21, all BEM notation with double dashes will no longer be available in SLDS 2.13.0 and later versions. For your styles to continue working with SLDS 2.13.0 and later versions, replace -- with _ in your CSS. For example, replace slds-button--brand with slds-button_brand. If you’re using SLDS 2.12.2 and earlier versions, your styles continue to work as no changes have been made to those versions.