This question already has answers here:
Difference between ID and Class
(8 answers)
Closed 8 years ago.
I want to know what is the exact difference between id and class in html and css.
I read somewhere that ids are unique and there can be only one element with corresponding id, and classes can be applied to many elements.
But I think this is wrong I have a code in which I have applied a single id to two elements.
<link type="text/css" href="main.css" rel="stylesheet">
</head>
<body id="header">
<p id="header2">Poster</p>
<p id="header2">Hello World</p>
</body>
</html>
and here is my css code
p#header2{
background-color:white;
width:50%;
height:50%;
margin-left:25%;
margin-right:25%;
text-align:center;
font-size:50px;
}
Could anyone please explain me???
Here is the Explanation in detail.
Use a class when you want to consistently style multiple elements throughout the page/site. Use the ID when you have a single element on the page that will take the style. class is a type of item and id is the unique name of an item.
EDIT:
CSS doesn't care
Regarding CSS, there is nothing you can do with an ID that you can't do with a Class and vice versa. I remember when I was first learning CSS and I was having a problem, sometimes I would try and troubleshoot by switching around these values. Nope. CSS doesn't care.
Javascript cares
JavaScript people are already probably more in tune with the differences between classes and ID's. JavaScript depends on there being only one page element with any particular, or else the commonly used getElementById function wouldn't be dependable. For those familiar with jQuery, you know how easy it is to add and remove classes to page elements. It is a native and built in function of jQuery. Notice how no such function exists for ID's. It is not the responsibility of JavaScript to manipulate these values, it would cause more problems than it would be worth.
EDIT2:
The Documentation clearly shows that HTML required the ID attribute to be unique in a page:
This attribute assigns a name to an element. This name must be unique in a document. If you have several elements with the same ID, your HTML is not valid.
So in JavaScript, getElementById() should only ever return one element. You can't make it return multiple elements.
Well, you can have more than one element with the same ID, but you shouldn't - because the consequences of doing so is unpredictable, due to differences between browsers.
ID: Id name must have 1.
<link type="text/css" href="main.css" rel="stylesheet">
</head>
<body id="header"> // ID
<p id="header2">Poster</p>
<p id="header2">Hello World</p>
</body>
</html>
Class: have multi tag using the same class name. And you can apply style for all class have name is "name".
<p class = "name">abcdef</p>
<image class = "name">......</image>
It's right:
The id of an element should be unique in the entire dom.
The class can be applied to multiple elements.
In css if you use this syntax
.yourClass {
color: white;
}
every element with class="yourClass" have the style color: white.
The id HAVE to be unique for different reason. The thing first jump into my mind its the use of jquery where you identify dom elements with the ID
The id attribute is meant to be unique and should contain a single identifier. The class attribute may contain a space-separated list of class identifiers allowing for cumulative application of css rules.
This is notwithstanding to css engines that do not complain on double ids.
Simple - id is the one that can be used only once and it will be unique. Scripts mostly use id's but it doesn't mean that id will only be used with js and not css. You can used a id as selector to in css.
While classes when two or more divs have the same styling so use class instead of increasing lines of codes.
Related
Consider a component c with html
<form id="someID"[formGroup]="codeEditorForm" novalidate>
</form>
with css
#SomeID{
color:grey;
}
If C gets included in a parent component P two times then wouldn't the id get duplicated as well which should not happen in html
Question 1 - should I not use id in Angular?
Question 2 - how shall I then write css rules without using id?
You can use IDs but generally you don't need to do it.
Angular scopes the styles automatically, so a class defined in one component won't affect a class with the same name in another component.
<form id="someID" class="my-form" [formGroup]="codeEditorForm" novalidate>
</form>
.my-form {
color: green;
}
Also note that you should make sure that there is always a space after you define a property. In your posted example there was no space after someId" Maybe it doesn't matter in this case but I guess it's better to start looking out for small things like this right away.
As far as I know there are there are two differences between id and class.
First is that id is used for single specific element, while class can target multiple elements, this means that you normally would use id selector if you are sure that an element will not be repeated, something like navbar.
And the second one is that id selector is more specific than class selector, this means that if there are any conflicts of styles, rules written in id selector override rules written in class selector
Normally you would not use it to override rules as there is other way to do it, so in order to decide which one to use all you need to do is think about this >> is the element you are giving id to really gonna be unique?
Angular support id on html. But as you are loading multiple times the 'C' component on your 'P' component,so the Id will be duplicated. So better you use class instead of id on your html..
<form class="yourClass"[formGroup]="codeEditorForm" novalidate>
</form>
On your 'C' component's css,
.yourClass{
color:grey;
}
So I want to select the <div> element with the class .thisClass, but not any other elements with class of .thisClass:
<div class="thisClass"></div>
<p class="thisClass"></p>
CSS Selector by class name and tag: div.thisClass { ... }:
div.thisClass {
background-color: red;
}
<div class="thisClass">thisClass (div)</div>
<p class="thisClass">thisClass (p)</p>
But this is a bad way to write selectors:
Don’t qualify class rules with tag names
The previous concept also applies here. Though classes can be used many times on the same page, they are still more unique than a tag.
One convention you can use is to include the tag name in the class name. However, this may cost some flexibility; if design changes are made to the tag, the class names must be changed as well. (It’s best to choose strictly semantic names, as such flexibility is one of the aims of separate stylesheets.)
BAD
treecell.indented {…}
GOOD
.treecell-indented {…}
BEST
.hierarchy-deep {…}
Using JavaScript
document.querySelector('div.thisClass')
Using jQuery
$("div.thisClass")
Using CSS:
<style>
div.thisClass{}
</style>
The following code illustrates how to select the first class from the list of classes in both CSS and Javascript.
CSS
.thisClass:first-child {
/*css property*/
}
JAVASCRIPT:
var divElement = document.getElementsByClassName('thisClass')[0];
i'm new to html/css and completed the css/html tutorial on codeacademy. I've reread over everything i've done and have a good knowledge around why everything works the way it does except the class/id tag. If i understand correctly the class tag is useful for when a bunch of elements should all recieve the same styling and id's are useful when you have exactly one element that should receive it's own styling. The thing i don't get is then what is the point of the id tag if i can get the same result using the class tag. For e.g. i have boxes 1,2 and 3 and i want them all to be the color black aka all recieve the same styling i would use the class tag. But i change my mind and now i want box2 to be white so in theory i should change box2 to an id tag so it can receive it's own styling but the thing is i can still use the class tag and get the same result by typing .box2 color:white;
My question is what is the point of the class and id tag if i can do the same thing for both using just the class tag.
Sorry if this is a difficult question to understand. I tried to word it as best as i could.
Ids are unique, you use id only for one element
<div id="me"></div>
On the other hand classes can be used to target more than one element
<div class="book">The Alchemist</div>
<div class="book">Harry Potter</div>
id tag is actually utilized the best in JavaScript, it is used to identify a tag uniquely among a bunch of tags. You would realise the importance of id tag when you start working with JavaScripts.
Suppose you have around 50 <p> tags in your HTML code. But you want to get value of one particular <p> tag, then the obvious way to do this is making you use of id.
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = Date();
</script>
The above code checks for the tag whose id is "demo" and then assigns the output returned by Date( ) to that corresponding tag.
id is for single element.
class is for group.
if you want to change color of box2. you can alse give inline css on them because inline css priority is higher then another type
This question already has answers here:
Several elements with the same ID responding to one CSS ID selector
(3 answers)
Closed 9 years ago.
I know the differences between ID and Class. But why both IDs take effect in this html? ID should be used uniquely, right?
#text {
text-align:center;
color: red;
}
Then id="text" was used twice in my same html page, and both get same effect. Why do i have to use "class", if 'id' has same effect?
That IDs seem to behave like classes when you have duplicate IDs in a page is nothing more than a side effect of how browsers implement CSS.
Whether a document conforms to its own rule that IDs should be unique is not relevant to CSS. CSS simply says to match elements that have a specific value in its ID attribute, to an ID selector:
An ID selector represents an element instance that has an identifier that matches the identifier in the ID selector.
It does not say anything about what should happen if there is more than one element with such an ID, because it assumes that you are working with a conforming HTML document.
You should use classes to group your elements because that's what HTML says to do, and you are trying to conform to valid HTML. Having duplicate IDs is quite simply not allowed by the HTML spec, and can result in unexpected behavior, so it is not something to be relied on regardless of what effect it has in browsers.
Yes, 'id' have the same effect like 'class'. But there is some different between them.
1. The 'id' selector can't be repeat in the same page. That means when there is a id selector
named "header",there can't be another "header" in this page.This is for js dom command.
2. The 'id' selector have a higher priorty. That means when you have a markup like this:
<style>
#header{color:red;}
.header{color:blue;}
</style>
<div id="header" class="header">
test
</div>
the color in test is "red". http://www.impressivewebs.com/difference-class-id-css/
the DOM will not fail to load with duplicate IDs, but you will have negative effects with other aspects, like javascript. If you are only using ID to identify where CSS rules should apply then they will work just like Class.
In general, ID should always be unique on the page, but the DOM will not enforce this.
IDs may work for css in some browsers, but not in others. There may also be bugs and side effects, for example:
<div>
<input type="radio" id="id1" name="r1" />
<label for="id1">this is radio label for id1</label>
</div>
<div>
<input type="radio" id="id1" name="r1" />
<label for="id1">this is another label for id1</label>
</div>
<!--the second label will also trigger the first radio change -->
Sometimes the functions won't be bind to your radio. Just keep IDs unique on the page.
This question already has answers here:
What's the difference between an id and a class?
(17 answers)
Closed 9 years ago.
I am learning HTML. Can someone please tell me what is the difference between class and id and when to use one over the other? They seem to do the same thing
<!DOCTYPE HTML>
<html>
<head>
<style>
#mycolor1 {color: red;}
.mycolor2 {color: red;}
</style>
</head>
<body>
<div id="mycolor1"> hello world </div>
<div class="mycolor2"> hello world </div>
</body>
</html>
They do not do the same thing.id is used to target a specific element, classname can be used to target multiple elements.
Example:
<div id="mycolor1" class="mycolor2"> hello world </div>
<div class="mycolor2"> hello world2 </div>
<div class="mycolor2"> hello world3 </div>
Now, you can refer all the divs with classname mycolor2 at once using
.mycolor2{ color: red } //for example - in css
This would set all nodes with class mycolor2 to red.
However, if you want to set specifically mycolor1 to blue , you can target it specifically like this:
#mycolor1{ color: blue; }
Read the spec for the attributes and for CSS.
id must be unique. class does not have to be
id has higher (highest!) specificity in CSS
Elements can have multiple non-ordinal classes (separated by spaces), but only one id
It is faster to select an element by it's ID when querying the DOM
id can be used as an anchor target (using the fragment of the request) for any element. name only works with anchors (<a>)
Classes should be used when you have multiple similar elements.
Ex: Many div's displaying song lyrics, you could assign them a class of lyrics since they are all similar.
ID's must be unique! They are used to target specific elements
Ex: An input for a users email could have the ID txtEmail -- No other element should have this ID.
The object itself will not change. The main difference between these 2 keyword is the use:
The ID is usually single in the page
The class can have one or many occurences
In the CSS or Javascript files:
The ID will be accessed by the character #
The class will be accessed by the character .
To put it simnply: id is unique to just one element in the whole HTML document, but class can be added to numerous elements.
Also, ID properties have priority over class properties.
ids and classes are especially useful if you plan on using javascript or any of its frameworks.
class is used when u want to set properties for a group of elements, but id can be set for only one element.
ID's must be unique (only be given to one element in the DOM at a time), whereas classes don't have to be. You've already discovered the CSS . class and # ID prefixes, so that's pretty much it.
ID provides a unique indentifier for the element, in case you want to manipulate it in JavaScript. The class attribute can be used to treat a group of HTML elements the same, particularly in regards to fonts, colors and other style properties...
ID is suitable for the elements which appears only once
Like
Logo sidebar container
And Class is suitable for the elements which has same UI but they can be appear more than once.
Like
.feed in the #feeds Container
the Id selector is used when referring to a single unique element.
The Class selector referrers a group of elements.
For example, if you have a multiple buttons that look the same, you should use class="mybutton" to have consistent styling.
In terms of performance:
CSS selectors are matched from right to left.
Therefore, .myClass should be "faster" than #myID because it misses out testing.
The performance speed is negligible for normally sized pages and you will probably never notice a difference so it's mostly just about convention.
Here is more info on why css is browsers match css selectors from right to left