Currently sharing on Facebook, Whatsapp and other social networks displays a nice image, however on Twitter nothing happens as we haven't opted in to support their service.
Twitter uses og tags but requires the following tag to show we've opted in. Shall we add it?
<meta name="twitter:card" content="summary_large_image">
Update: A mini- reverse-engineering experiment shows that Twitter can parse cards on a page from pure og meta tags, we don't need to add a twitter:card. Also, og:description summaries are optional and will be implemented in separate task T213505, so we can deploy an interim set of tags even before the description is ready.