Why h1#chapter1 and not simply #chapter1?

[Mike3456]

The value of id attribute is supposed to be unique. It doesn't make any sense to use a CSS selector with an element name in front of #id_name, in the sense that no two elements can have the same id value. W3C website implies that two different elements can have the same id. Theoretically, there is a special case for h1#chapter1 where there is a different element with id=chapter1 but in such case there could not be h1 with id=chapter1 in the same document anyway. Such special cases have to be described in any spec and not left out to speculations

 

This excerpt is from

 

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https://www.w3.org/TR/css3-selectors/#id-selectors:

 

The following ID selector represents an h1 element whose ID-typed attribute has the value "chapter1":

h1#chapter1

The following ID selector represents any element whose ID-typed attribute has the value "chapter1":

#chapter1

The following selector represents any element whose ID-typed attribute has the value "z98y".

*#z98y

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Edited July 20, 2016 by Mike3456

[dsonesuk]

It easily gives you the element type the id is applied to, so you can group these element type making easier to maintain maybe?

[Ingolme]

The ID is unique within the page. If, for any reason, you don't want to select an element that's not an <ht> you can use h1#chapter1.

 

Usually you won't need to, but the example is there to show it's allowed.

 

IDs may be unique within a page, but one same stylesheet can apply to many different pages. Perhaps on some of the pages #chapter1 is not an <h1> element.