Quelques attributs HTML oubliés mais qui peuvent s'avérer utiles :
'reverse' pour les listes, 'download' pour un <a>, 'title' pour les feuilles CSS…

The enterkeyhint Attribute For Virtual Keyboards
The title Attribute On Stylesheets
The cite Attribute For The <blockquote> And <q> Elements
Attributes For Custom Ordered Lists
The download Attribute For The <a> Element
The decoding Attribute For The <img> Element
The loading Attribute For The <iframe> Element
The form Attribute For Form Fields
The cite And datetime Attributes For Deletions/Insertions
The label Attribute For The <optgroup> Element
The imagesizes And imagesrcset Attributes For Preloading Responsive Images