blocks.getBlockDefaultClassName filter is not applied to heading, paragraph and image blocks.

[hatsumatsu]

Describe the bug
The filter blocks.getBlockDefaultClassName does not alter the class attribute of heading, paragraph and image block.

To reproduce
Steps to reproduce the behavior:

1. Add the filter as decribed in the docs

function addBlockCustomClassName( className ) {
    return 'wp-block ' + className;
}

// Adding the filter
wp.hooks.addFilter(
    'blocks.getBlockDefaultClassName',
    'my-plugin/add-block-custom-class-name',
    addBlockCustomClassName
);

2. Class attribute of blocks like columns, group etc. is altered correctly but paragraph, heading and image blocks are untouched.

Expected behavior
All block should be affected by the filter.

[youknowriad]

That's not a bug, these blocks opt-out of the default className.

[hatsumatsu]

@youknowriad okay, but why? In the editor these are treated as and called blocks. Shouldn't it be possible to target all blocks in CSS via a generic class?

Coming from #6639 it seems that both workarounds to easily target all blocks via one CSS selector don't work.

[youknowriad]

okay, but why? In the editor these are treated as and called blocks

Because they are blocks, a block is not necessary something with a given class. A block can even be multiple DOM elements without any wrapper. A block doesn't map to a DOM element, it's just an entity that wrappes a fragment of content and allows you to edit it visually.

In the case of paragraphs, headings and images we decided to opt-out of these classNames because these blocks are too common and we don't want to pollute the post-content with these.

I hope that answers your questions.

[hatsumatsu]

Okay, thanks for clarifying. Guess the expectations of what the desired markup looks like differ based on different frontend workflows. I was just hoping for a clean BEM-like code style...