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4. Class and ID

Classes and ID allows to attach a label to HTML elements. This label can be used in the CSS to refine the selection of a subset of elements.

ID

ID are used to identify a unique element (The ID should be unique in the entire webpage). For instance an element <p> with an id label is defined in the HTML as

<p id="label"> Some text </p>

Specific ID can then be selected in the CSS code using #label ex.

#label {
        color: red;
}

Classes

Classes are used to identify a subset of element, and at the opposite of ID, may be reused on different elements. Example of <p> and <h1> elements with a class

<h1 class="label"> My title </h1>
<p class="label"> My text </p>

Classes are selected in the CSS using .label. ex.

.label {
        color: red;
}

  • Note: you can define several classes for one HTML element (ex. class='label1 label2').

Application

Consider the following code

<p class="c1"> Value A </p>
<p class="c2"> Value B </p>
<p class="c1"> Value C </p>
<p class="c1 c2"> Value D </p>
<p> Value E </p>

And this incomplete CSS

p {
        margin: 10px;
        padding: 10px;
}
XX {
        background-color: lightgray;
}
YY {
        font-weight: bold;
}

  • Set XX, and YY in order to obtain this result

classes exercise

Selectors with classes

Classes can be combined with names of elements to refine the selection. Given an html element e and classes c, the CSS selector e.l selects only elements e with the class l.

Note: It is usually useless to refine the selection of an ID, as it defines a unique element.

Examples (make sure to understand well their differences):

  • e.label: Select e elements with the class label.

  • e, .label: Select all e elements, and all elements with class label.

  • p .label: Select elements which are children of p and have a class label.

Application: Explain in full text which elements would be selected by these selectors

h1.main, .second
h2 .main, #selected, p em.red
body p.selected strong

Pseudo classes

Pseudo-classes are used to define a state of an element, for instance to indicate when the user pass the mouse over the element. Pseudo classes allow CSS to interact (in a limited way) with the action of the user.

Pseudo classes are used by default on link element <a> that change color when the mouse passes over it, and once the link has been visited. Pseudo classes can however be used on any elements using CSS.

  • Experiment with the following examples

HTML

<a href="https://www.polytechnique.edu/"> Link </a>

CSS

body {
        text-align: center;
        font-size: 200%;

}
a {
        color: blue;
        text-decoration: none; /* take away the default underline */
}

/* The mouse is over the element */
a:hover {
        color: red;
        font-weight: bold;
}

/* The mouse click on the element */
a:active {
        background-color: black;
}

Exercises

Menu

Considering the following HTML body code to model a menu, create the associated CSS that mimics the following appearance.

<a id="selected" href=""> Home </a>
<a href=""> News </a>
<a href=""> Contact </a>
<a href=""> About </a>

Note the change of color when the mouse pass over an entry of the menu

Dynamic buttons

Create a webpage that reproduce the behavior of following one Note the behavior of the boxes when you pass and click on the boxes

Hidden element

Observe the behavior obtained when passing the mouse over the following text and reproduce it using pseudo classes.

Hints:

  • display:none; allow to hide an element

  • e1+e2 is a CSS selector allowing to select the element e2 immediately following the element e1 in the document hierarchy.