Tabindex

Element is focusable, but not reached with keyboard

<div tabindex="-1">Howdy</div>

Element is focusable with the keyboard, it's order is defined by the document structure

<div tabindex="0">Howdy</div>

Accordion markup

Accordion Markup - Sara Soueidan

<details>
  <summary>What is the population of New Orleans?</summary>
  According to 2010 Census Bureau estimates, New Orleans' population is made up of approximately 343,829 residents.
</details>
details[open] summary {
    color: red;
}

Defining acronyms & abbreviations

dfn is the defining instance of a term (with the term being deinfed/explained in it's nearest parent). It can be used in conjunction with the abbr tag which defines an abbreviation or an acronym.

<p>The <dfn><abbr title="International Space Station">ISS</abbr></dfn> is a space station in low earth orbit.
</p>