Snippets to list all custom elements on the page.

index.js

customElsMap = Array.from( document.querySelectorAll( '*' ) ).filter( (e) => e.tagName.includes('-'))
    .reduce((accu, el)=>{ 
        let set = accu.get( el.tagName );
        if( ! set ) {
            set = new Set();
            accu.set( el.tagName, set );
        }
        set.add( el );
        return accu;
    }, new Map());

customElsMap = Array.from( document.querySelectorAll( '*' ) ).filter( (e) => e.tagName.includes('-'))
    .reduce((accu, el)=>{ 
        let set = accu[ el.tagName ];
        if( ! set ) {
            set = [];
            accu[ el.tagName ] = set ;
        }
        set.push( el );
        return accu;
    }, {});