by Niall Kennedy on WordPress.org

Twitter Card display on desktop
Generate inline content previews on Twitter.com and Twitter clients by including Twitter Card HTML meta elements for individual posts.
All cards are a Twitter Card summary by default. Tap into the twitter_card_properties filter to reference the Twitter accounts of your site or author.
Test your site’s Twitter Card display through Twitter’s card preview tool.
twitter_cards_properties – act on an array of properties before they are output to the pagetwitter_cards_htmlxml – override the default treatment of html with xml. XML will self-close the meta void element ( “” vs. “” )