It's not possible to define HTML attributes without value

[msamsel]

There is few value-less attributes in HTML which cannot be created with a writer.
e.g. elements as below cannot be created with attributes such as download or required.

<a download href="…">…</a>
<input required type="text">

All elements get values:
https://github.com/ckeditor/ckeditor5-engine/blob/da5670ac2a18186111005cb243f46f9fcfb28b2f/src/view/element.js#L772-L788

[Reinmar]

While this is a missing feature (not really a bug), I'd wonder how important it is taken that required equals required="" (or something similar – can't remember the alternatives). Of course, from a purist's POV, a bare required is better, but it's just a data format, not an art, so from functionality perspective required="" is fine too.

[mlewand]

The API might be tricky to do this in a way that is compatible with current design. One idea that comes to my mind is to create a dedicated type/value that would be used for that. Something like this:

import EmptyAttribute from 'xyz';

myElement.setAttribute( 'required', EmptyAttribute );

[bbenjamin]

I need to be able to set alt="" with the writer. I've tried the following:
writer.setAttribute('alt', true, viewElement);
writer.setAttribute('alt', null, viewElement);
writer.setAttribute('alt', '', viewElement);

The resulting markup is <img alt="""" /> for each of these, but I need for it to be an empty set of quotes so there's a way to designate an image as decorative and should be skipped by assitive technology. The alt="""" results in it being acknowledged by screenreaders, even thought the tag value has no alphanumeric characters.

[Reinmar]

@bbenjamin I tried to reproduce the problem that you're describing but all seems to work fine for me.

For instance, I made Paragrap's downcast to look like this:

		editor.conversion.for( 'downcast' ).elementToElement( {
			model: 'paragraph',
			view: ( modelElement, { writer } ) => {
				const p = writer.createContainerElement( 'p' );

				writer.setAttribute( 'data-foo', '', p );

				return p;
			}
		} );

And the DOM looks good:

The data returned by editor.getData() looks good too:

<h2>Heading 1</h2><p data-foo="">Paragraph</p><p data-foo=""><strong>Bold</strong> <i>Italic</i> <a href="https://ckeditor.com">Link</a></p><ul><li>UL List item 1</li><li>UL List item 2</li></ul><ol><li>OL List item 1</li><li>OL List item 2</li></ol><figure class="image image-style-side"><img src="sample.jpg" alt="bar"><figcaption>Caption</figcaption></figure><blockquote><p data-foo="">Quote</p><ul><li>Quoted UL List item 1</li><li>Quoted UL List item 2</li></ul><p data-foo="">Quote</p></blockquote>'

I also checked that you can have images with empty alt:

<figure class="image image-style-side"><img src="sample.jpg" alt=""><figcaption>Caption</figcaption></figure>

Just to be 100% sure, I changed the value of this image's alt by the model writer (ensuring that the entire pipeline from the model to the view works fine):

editor.model.change( writer => { writer.setAttribute( 'alt', '', editor.model.document.getRoot().getChild( 0 ) ) } );

So, I'm afraid the issue is somewhere outside CKE5's code. One thing I'd check for sure is the custom HTML writer that you use.