Out of curiosity, I probably went through all the html tags currently in existence and came across some that are less used or that I have never heard of or used personally, but which I thought would be very useful if they were used more widely.

Here is the list of tags that I found very useful to be used in many situations, some even specific for certain applications

• <output> Perform a calculation and show the result in an <output> element
• <details> Specify details that the user can open and close on demand
• <samp> Define some text as sample output from a computer program in a document
• <optgroup> The optgroup tag is used to group related options in a drop-down list
• <meter> The meter element is used to display a gauge
• <mark> Highlight parts of a text
• <datalist> A datalist with pre-defined options (connected to an <input> element)
• <base> Specify a default URL and a default target for all links on a page
• <abbr> Defines an abbreviation or an acronym