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Coding Interview Prep: Let's Test Your CSS
by Stas Melnikov December 26th, 2023
Too Long; Didn't Read
A brief test of some CSS skills to help prospecting developers prepare for interviews or just to keep their skills sharp.Company Mentioned
CSS is a language with a bunch of topics. There is a developer who knows all unlikely. Honestly, we don't need to know it to do our job. But there is a piece of CSS without that we can't do. It is the goal of my questions.
If you can't answer some questions, don't worry. It means you have a chance to level up your knowledge 😉
What will the specificity be of the following selector?
:is(#container, .content, main) {
color: red;
}
The :is() pseudo-class function helps browsers select the highest from a given selectors list. In our example, a higher selector is #container. The specificity of this selector is 0, 1, 0, 0. It will be used for the whole at-rule.
The computed value of the color property is red. True or false?
<body>
<span id="container" class="container">content</span>
</body>
.container {
color: red;
}
:where(#container) {
color: blue;
}
True. The :where() pseudo-class function nulles specificity. So, the .label selector has more specificity. It's why the computed value of the color property is red.
What is the computed value of the background-color property?
<body>
<div id="container" class="container">content</div>
</body>
@layer basic, components;
.container {
width: 1rem;
height: 1rem;
}
@layer components {
.container {
background-color: pink;
}
}
@layer basic {
#container {
background-color: blue;
}
}
Layers are defined in order of priority. The last is more high. So the answer is pink.
What is the mistake in the following code?
.container {
display: inline;
width: 1rem;
height: 1rem;
}
If the element has display: inline the width and height properties don't have an effect. So we shouldn't define them.
What is the algorithm for calculating the computed value of the width property of the .child element?
<body>
<!-- case #1 -->
<div class="parent">
<div class="child">content</div>
<div class="child">content</div>
</div>
<!-- case #2 -->
<div class="parent parent-flex">
<div class="child">content</div>
<div class="child">content</div>
</div>
</body>
.parent {
display: block;
}
.parent-flex {
display: flex;
}
In the case #1, the .child elements are block-level elements. Their width property is equal to the width property of the parent element.
In the case #2, the .child elements are flex items. Their width property is calculated depending on content.
What is the computed value of the display property of the pseudo-elements?
.parent {
display: inline-grid;
}
.parent::before {
content: "";
display: inline;
}
.parent::after {
content: "";
display: flex;
}
block and flex. The grid or inline-grid values transform inline-* values of the display property of the child elements to block alternatives.
.parent {
display: inline-grid;
}
.parent::before {
content: "";
display: inline; /* display: block will be here */
}
.parent::after {
content: "";
display: flex; /* display: flex will be here */
}
What is the difference between the default position of the child elements in the case of the parent element with display: flex and in the case with display: grid?
The child elements inside the parent element with display: flex display one by one in line. In contrast, the elements will be displayed one below the other in the case with display: grid.
What is the computed value of the width and height properties of the .child elements?
<body>
<div class="parent">
<div class="child">content</div>
<div class="child">content</div>
</div>
</body>
.parent {
display: grid;
width: 100rem;
height: 20rem;
}
The width property of the .child element is equal to the width property of the parent element. So, the computed value of the width property is 100rem, i.e 100 * 16 = 1600px.
The height property of the child element inside of the parent with display: grid fills all space. If the parent has a few items, space will be shared between them equally. So, the computed value of the height property of the child element is 20rem / 2 = 10rem, i.e 10 * 16 = 160px.
I use 16px like a browser's default font size.
The margins of the .child element end up outside of the parent element in all cases. True or false?
<body>
<div class="parent">
<div class="child">content</div>
</div>
</body>
/* case #1 */
.parent {
display: inline-flex;
}
.child {
display: block;
margin-block: 1rem;
}
/* case #2 */
.parent {
display: grid;
}
.child {
display: block;
margin-block: 1rem;
}
False. Margins of the child elements don't end up outside the parent element with display: flex, display: inline-flex, display: grid and display: inline-grid.
Does margin collapsing work inside elements with display: inline-flex and display: inline-grid?
No, it doesn't work. Margins will be summed up inside of the element with display: flex, display: inline-flex, display: grid and display: inline-grid.
The position of the pseudo-element is centered horizontally and vertically. True or false?
.container {
display: grid;
height: 100dvh;
}
.container::before {
content: "";
width: 1rem;
height: 1rem;
margin: auto;
}
True. Browsers will share all space between the childs and the parent's borders evenly.
What is the computed value of the min-width property?
<body>
<div class="parent">
<div class="child">content</div>
</div>
</body>
body {
display: block;
}
.parent {
display: grid;
/* min-width: ? */
}
.child {
/* min-width: ? */
}
The initial min-width value is auto. So the computed min-width value of the .child element is auto.
But if the block, inline, inline-block, table or table-* value is defined for the element the computed min-width value of its child elements is 0.
body {
display: block;
}
.parent {
display: grid;
/* min-width: 0 */
}
.child {
/* min-width: auto */
}
How can we use the gap property to replace the margin property?
.parent {
display: inline-flex;
}
.parent::before,
.parent::after {
content: "";
width: 1rem;
height: 1rem;
background-color: #222;
}
.parent::before {
margin-right: 1rem;
}
.parent::after {
margin-left: 1rem;
}
We should define the gap property for the .parent element.
.parent {
display: inline-flex;
gap: 1rem;
}
.parent::before,
.parent::after {
content: "";
width: 1rem;
height: 1rem;
background-color: #22;
}
The computed value of the display property is block. True or false?
.container {
position: absolute;
display: inline;
}
True. If the absolute or fixed value is defined, browsers will transform all inline-* values of the display property to block alternatives.
.container {
position: absolute;
display: inline; /* display: block will be here */
}
Why is the computed value of the height property of the .parent element equal to 0?
<body>
<div class="parent">
<div class="child">content</div>
</div>
</body>
.child {
position: fixed;
}
The element with position: absolute or position: fixed is removed from the normal document flow. So, the parent elements don't see it. It's why the computed value of the height property is 0.
What does the isolation property do in the following example?
<body>
<div class="parent">
<div class="child">
<span>content</span>
</div>
</div>
</body>
.parent {
background-color: purple;
}
.child {
position: relative;
isolation: isolate;
}
.child::after {
content: "";
background-color: green;
position: absolute;
inset: 0;
z-index: -1;
}
We should remember which stacking context is used by browsers when using the z-index property.
By default, a root-stacking context is the html element. It's why the pseudo-element is behind the .parent element without isolation: isolate.
We create a new stacking context with the isolation property for the .child element. So, the pseudo-element displays behind the text but in front of the .parent element.
What is the position of the pseudo-element?
.container {
display: grid;
place-items: center;
position: relative;
height: 100dvh;
}
.container::before {
content: "";
width: 1rem;
height: 1rem;
position: absolute;
bottom: 0;
}
First, the pseudo-elements are displayed in the center because place-items: center is applied.
It shifts by Y axis to the bottom parent border after position: absolute, bottom: 0 are applied because the top, right, bottom and left properties are more priority than the place-items property.
Might the computed value of the flex-basis property be more than the computed value of the max-width property?
The flex-basis property has priority over the width property, but its value must also be in the range of values of the min-width and max-width properties. So the answer is no, it can't.
What is the computed value of the padding property?
:root {
--padding-vertical-start: 1rem;
--padding-horizontal-end: 2rem;
--padding-vertical-end: 3rem;
}
.container {
padding: var(--padding-vertical-start)
var(--padding-horizontal-end)
var(--padding-vertical-end)
var(--padding-horizontal-start);
}
We should define all parts of the shorthand when using CSS Custom Properties. If we don't make it, browsers can't apply values.
It happens in our example. The padding shorthand requires four values. But the developer-defined only 3. Browsers can't set paddings. So, the computed value is 0.
Why will the computed value of the background-color property be green for the p element?
body {
background-color: green;
}
p {
--background-color: inherit;
background-color: var(--background-color, inherit);
}
A CSS custom property inherits a value from the same custom property defined for parent elements. If a custom property is omitted, browsers will use fallback.
In our example, the --background-color property is omitted from parent elements. So browsers use the fallback, i.e. the inherit keyword that inherits the green value from the background-color property of the body element.
Make the scroll-behavior property safe with vestibular motion disorders.
html {
scroll-behavior: smooth;
}
We should wrap the code using the prefers-reduced-motion media feature. It'll help to display smooth scrolling only if users allow it in OS settings.
@media (prefers-reduced-motion: no-preference) {
html {
scroll-behavior: smooth;
}
}
What is the computed value of the font-size property?
html {
font-size: calc(1rem + 1px);
}
Default browser font size is 16px in most cases. If it isn't changed, the computed value of the font-size property will be 17px.
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