Are there any restrictions for using HTML tags in "html" field?

According to https://developers.google.com/glass/v1/reference/timeline#html one can use HTML.

Are there any restrictions for using HTML tags in "html" field?

We will update the documentation regarding this shortly but in the meantime, here are the restrictions:

• Allowed HTML elements: Use these elements in your timeline cards.
  • Headers: h1, h2, h3, h4, h5, h6
  • Images: img
  • Lists: li, ol, ul
  • HTML5 semantic: article, aside, details, figure, figcaption, footer, header, nav, section, summary, time
  • Structural: blockquote, br, div, hr, p, span
  • Style: b, big, center, em, i, u, s, small, strike, strong, style, sub, sup
  • Tables: table, tbody, td, tfoot, th, thead, tr
• Blocked HTML elements: These elements, and their contents, are removed from html payloads.
  • Document headers: head, title
  • Embeds: audio, embed, object, source, video
  • Frames: frame, frameset
  • Scripting: applet, script
• Other elements: Any elements not listed above are removed, but their contents are preserved.