Authoring
Visual editor or raw code
Each block's HTML field can switch between two modes, as you prefer:
Visual editor (TinyMCE)
PrestaShop's familiar WYSIWYG interface, for composing without writing tags.
Raw code
A code editor with syntax highlighting and line numbers (HTML, CSS, JS), which reflects the field's content 1:1 without reformatting or touching spacing.
A block's CSS and JS, when filled in, are never injected inline into the page: they are written to a static file dedicated to that block and loaded through PrestaShop's normal asset pipeline, combined and minified with the rest of the site.