Your elements
Definition
What the merchant picks from the Add block menu.
Must start with custom_. Cannot be changed once blocks are using it.
{{ key }} inserts a value, {{{ key }}} inserts it as markup,
{% if key %}…{% endif %} hides a part when empty, and
{% each key %}…{{ . }}…{% endeach %} repeats for a list field.
Scoped to this element. Bare declarations style the block itself; nested selectors apply inside it.
Rendered with sample values, through the same template engine and sanitiser the storefront uses.