Disregarding speed and compatibility, why not use only classes and never use IDs in HTML/CSS? [closed]_问答_开发者

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I use only classes and never use IDs. Many people like to use IDs for different reasons.

I've seen many questions regarding IDs vs classes on stackoverflow, but no one addressed pure code organization point of view disregarding compatibility and runtime performance.

From code organization point of view, I think that using IDs is bad just like using global variables in Visual Basic code.

One reason is that IDs have to be unique which introduces unnecessary and bad dependency between different independent parts of your code (controlling different parts of HTML DOM tree).

Another reason is that making new class names is actually easier than ID names because with IDs you have to worry about global scope and with class names you need to worry only about uniqueness in local scope, same benefit as with local variables.

Most people will argue that performance of addressing by ID is better than by class and I will agree with that. But as browsers become more advanced with native implementations of CSS addressing from javascript and computers become faster, performance becomes less and less important. So let's disregard it and concentrate only on organization of code in context of current question.

This discussion started here, but my potentially wrong advice generates negative points and became too big to keep in comments, so here I try to convert it into something positive and manageable.

One visible point in favor of IDs is to use them as a tool of rule prioritization because priority of #name is higher than priority of .name. My response: using IDs to raise priorities is bad hack, it's cleaner and there is more freedom if you use additional root elements inserted between body and other levels of tree, for example priority of body div div span.class1{} is higher than body div span.class1{} is higher than body span.class1{} is higher than span.class1{}. Another tool to use for that purpose is !important. S开发者_如何学编程ome may argue that using more root elements means more difficulties when the page structure changes, but I don't think this is the case because you never have to put anything between body and designated for prioritization divs. Those divs can always stay below body and above all other content.

Another interesting association was brought about pointers and that IDs are not bad because pointers are not bad. My response: pointers are bad if you hardcode absolute memory address in your code. Using relative pointers is always better (examples: using segments(CS,DS,SS,ES) in 8086 CPU; relative variable and method addresses generated by compilers). If we consider DOM tree as memory and compare using ID to using class then #name represents absolute memory address, but div.tab1 .name represents relative address (relative to div.tab1).

Another supporting point that I've seen for IDs is that elements with IDs are more easily available in javascript as becoming global properties. My response: again, this is like saying that global variables in Visual Basic are more conveniently available. The problem is that you can't keep large enough global (or any other) namespace in order without introducing naming hierarchy like level1_level2_name, which is just a hack to replace one namespace mechanism with another. DOM tree is convenient enough to organize namespaces, why disregard it ?

Namespace simulation inside IDs using underscore is bad because you can't establish naming context and will have to duplicate all paths everywhere in your code. That practically means that you won't be able to use CSS preprocessors that fix inability of CSS to use contexts.

quick answer: its a best practice, if you have only one instance of something and you only want one instance if it, use an ID to define that there should only be one instance of it.

This is like the difference between constant variables vs regular variables. You can use a regular variable to be a constant variable, but its better to define it as such if that is what its intended to be.

It lets fellow programmers (and yourself) know more information about the object.

I agree with you in general: Classes are much cleaner to use; you can create "namespaces" and clean cascades with them; and they can be combined: class='class1 class2'.

IDs still have their place when you're addressing really unique elements on the page, especially when addressing an element that is going to be changed in JavaScript later (e.g. a hidden overlay.)

I look at classes and ids the same way I look at a (programming) class versus an object. An object is one, specific instance of a class. I want all my classes to share a certain number of behaviors, but an individual object may have its own unique properties. CSS classes are for applying properties to broad groups of similar items, ids are for specific items and the specificity hierarchy reflects that.

To invert your question, why use classes at all when you could achieve the same effects with very specific tag selectors? For ease of use, repeatability and clarity of intent.

For me, I like using IDs on HTML elements that are absolutely unique, and classes on things that are possibly non-unique (regardless of whether or not they are).

For example, I would use <div id="header"> because there can only be one header. But I would use <div class="profile"> if there could conceivably be more than one on the page (even if there is only one). This makes the CSS a little easier for me to understand.

"DOM tree is convenient enough to organize namespaces, why disregard it ?"

Because the DOM can change due to AJAX or other javascripty-goodness.

I like @ocdcoder 's constant/variable analogy. Sometimes you want to refer to exactly that particular element. Having to adhere to a strict DOM namespace is a straightjacket that doesn't help maintenance at all, imho.

I think the discussion is incomplete without addressing the underlying reason for using classes and IDs. Saying that either should be used in every situation does not work well generally and historically.

The original purpose of classes was to attach presentation to the document, or introduce style as a separate concern than the structure of the document itself. Please correct me if I am wrong, but I think you are trying to address the problem of attaching semantic information to the elements rather than just style. If that is indeed the case, then classes serve two purposes for us - controlling presentation, and acting and semantic tags. IDs serve the purpose of acting as a unique semantic tag.

XML is highly extensible and allows namespaces which was supposed to be used in XHTML documents to assign meaning to a document as authors saw fit. But there were none in HTML and maybe it was browser incompatibilities or the non ease of use (as Tom mentioned), but most web pages and applications did not take the path of using namespaces in XHTML.

Surely the HTML spec authors saw the glaring need for attaching semantic data to HTML documents and introduced the data- attributes in HTML5 that could be assigned to any element. I would argue that if it's semantic meaning that you are concerned with, this is absolutely the best approach so far, but again browser incompatibilities have always had a major role in determining which spec becomes more commonplace and I hope IE does not have their say this time.

Using an id attribute allows you to link to an element E.g. if you’ve got this HTML:

<div id="log-in">

you can link to the log in section of the page using href="#log-in".

You’re quite right that classes are usually the most convenient and appropriate way to identify page components. But assuming that you’ve got an element that only appears once per page, and assuming you can give it an appropriate name or generate one, I don’t think you’re likely to run into problems using an id for it.