In Vaadin 20 I could add an icon as a Java component:
add(new Icon("vaadin", "check));
or in HTML:
<iron-icon icon=vaadin:check></iron-icon>
Changing nothing but the Vaadin version, in Vaadin 21, the first shows an icon, the second is blank. I've gone through the Vaadin 21 documentation and there's nothing I can see about a formatting change, what's the deal?
I have no experience on this, but…
See this Answer by Jean-Christophe Gueriaud.
He explains that as part of the transition away from Polymer, iron-icon is no longer available in Vaadin 21 and later. The replacement is vaadin-icon.
One user reports that annotating @Uses(Icon.class) while replacing iron-icon with vaadin-icon in the template makes it work.
Perhaps the new doc pages Images and Icons and Vaadin Icons might help.