When creating custom block, the inherit function build() return an element, usualy it include tag: #markup,

but it seems that it filter out the attributes, so how can we overwrite it correctly without the filter?

In other words we want to prevent drupal to filter out style attribute from custom block code

The '#markup' is filtered in ->ensureMarkupIsSafe(), which is called from within ->doRender().

The same does not apply to '#children' .

Drupal core provides a couple dozen different input #type elements that can be added to forms. This includes one for every standard HTML5 input element, and some Drupal-specific ones that encapsulate more complex interactions like uploading files. But how can you know what elements exist? Where do you find information about what Render API properties each element uses?