Heads, well written and otherwise

Open Graph

• An article — the article namespace in full: times, author, section, tags
• Several images — four og:image groups, each with its own width, height, alt and type
• Relative URLs — og:image and og:url written relative, which the protocol forbids
• Broken tags — empty content, repeated properties, values of ten thousand characters
• Declared with name= — og: properties on the wrong attribute, the way half the web ships them

Other vocabularies

• Twitter card — summary_large_image with player, app and label fields
• JSON-LD — an article, a breadcrumb trail and an organisation
• Microdata and RDFa — the same product described three ways

Directives

• Canonical elsewhere — points at another page on this site
• Canonical on another host — points at example.com
• Robots directives — noindex, nofollow, noarchive and a googlebot-only rule
• Meta refresh — a redirect written in the head
• A base element — every relative link on the page resolves somewhere else

Awkward documents

• hreflang — three languages pointing at each other
• Mixed content — an https page loading http resources
• Latin-1 — a page that is not UTF-8 and says so
• Unicode — emoji, right-to-left text and a title of combining characters
• No head at all — no title, no description, no charset