What's the point of writing div#id_name vs. #id_name? - css

Question

I've noticed that a lot of css style recipes are often stated like this:
div#id_name {
blah: blah;
}
But since IDs are unique, what's the point of sticking "div" in front of #id_name? Is there any advantage over the following snippet?
#id_name {
blah: blah;
}
I would argue that the latter is superior because you might decide to make the id_name element into something besides a div.

Answer 1

This is primarily done to advance specificity and to hint the document as to what type of element #id_name is.
First, specificity:
Specificity determines which styles are actually applied to your element. The more specific you are in calling your element out, the more priority that block of properties takes over another.
For example:
Given HTML
<div id="id_name">
Look at this blue text!
</div>
With CSS
div#id_name {
color: red;
}
#id_name {
color: blue;
}
Results in
This will render a div with red text as opposed to blue text. This is beneficial when writing a framework if you want to guard your styles from being arbitrarily overwritten by local styles.
Secondly, hinting:
Oftentimes, CSS is an afterthought. It's a shame, too, as it's gotten increasingly more powerful and has taken many of the responsibilities previously reserved for client-side scripting languages like JavaScript. There is no implicit inheritance in CSS, rather it's explicit via a long declaration.
What I'm talking about with this is that you don't see
div {
.my-class {
/* RULES! */
}
#my-id {
/* RULES! */
}
}
as a part of CSS unless you're using a precompiler like LESS or SASS. Hinting a document with the element name instead of only the id or class allows for much greater readability for not only future you, but any collaborators you may have on the project.
And finally:
Sometimes it just doesn't make sense to not add a an element guard to your rule. If I have a rule that sets things like height, width or padding, I wouldn't want that same rule applied to a span. I would rather see it fail loud than silent to prevent rules being applied that have no place being there. It can cause messy and unexpected results given the exact scenario you described.
In addition, there's no guarantee that #id-name won't be re-used on a later page for an element that is not a div in the scenario you gave. So there's that, too.

Answer 2

Using ID's have a very strict specificity issue. Realistically, according to the standards, you can only use an ID once in any given HTML document. That doesn't mean you can't use ID's as styling selectors, though, it does come with dangerous pitfalls in larger projects. They're fine if you're using them as targets in Javascript. Go crazy.
ID selectors are very, very specific in targeting elements and in return, you end up with problems later down the line dealing with CSS specificity. Class selectors are reusable and have much looser specificity. Styling with ID's doesn't have anything different that a class selector doesn't have, so why use them if they're causing specificity issues? Read this and this. They are both fantastic articles on why ID's are not cool for CSS. It is a personal preference, but, making your CSS very specific is a front-end disaster in all real-world cases web development.
So, to answer your question properly, adding div at the start of and id selector, like div#id_name means you can only apply that id to a <div> element. You couldn't add it to a <span>, or any other element for example, which is an insanely restrictive method of styling in CSS. If you just use #id_name, you can apply this selector on any element instead.

Answer 3

The only difference is that div#id_name has higher specificity. This is seldom relevant, and there are other ways to make a selector more specific. People may include the element (tag) name for documentation purposes, but then they take the risk that was referred to in the question: someone might change the div to, say, p and forget to modify the CSS selector(s).