Pure CSS syntax highlighter, no Javascript required. Includes built-in tooltips with UI themes and syntax highlighting themes.
You may also be interested in ...
HTMLrev
HTMLrev provides free HTML website templates built with vanilla CSS, Bootstrap, Tailwind, Angular, React, Vue, Nextjs, Nuxt, Svelte, Gatsby, Astro and Laravel. All templates are free for both personal and commercial projects including client work.
PaperCSS
The less formal CSS framework.
Shiny
JS library which simulating reflections on devices that support the DeviceMotion event.