Standard web components from the wider ecosystem can now live inside an LWC template as first-class elements.
Where
Lightning Web Component templates, via the lwc:external attribute on the element.
How
Mark the custom element with lwc:external and LWC treats it as an external web component, passing attributes and rendering it in place. No changes to the third-party component are needed and no iframe or static resource wrapper is required.
Why
Until now, reusing a web component built outside Salesforce meant wrapping it awkwardly or rebuilding it in LWC. This opens the door to design system components and vendor widgets with far less glue code.