Not sure when it happened, but the specification now allows you to leave quotes off of tags. <header id=top> or <link rel=stylesheet media=screen href=test.screen.css> Are back to being valid HTML. Why? Who knows. I never got a straight answer on why 4 Strict made them mandatory. https://deathshadow.medium.com/html-attribute-value-quotes-are-optional-again-9c619c52e991
I think it's bad tone. Maybe not all browsers will support it. In the future I will write as usual <header id='top'>
maybe it was to keep quotation marks from having to be modified for use in other coding languages <div class="myclass"> output += "<div class=\"myclass\">"; output += '<div class="myclass">'; etc.
I think it was always valid, the problems start with class="one two three". I guess it's just better practice to have double quotes for code readability. I have to add quotes to class and style, so it's easier to read , especially for code highlighters?