Top 11 Popular CSS Interview Questions

CSS, Cascading Style Sheets, is a style sheet language that is simple and easy to learn. It is basically about how to represent HTML elements on the screen. It is mainly used for adding styles to web pages. It is one of the favorites of developers and designers for adding styles to web pages. So, today we will be talking about the 11 most asked questions on CSS.

11 Most Asked Questions On CSS

1. How to disable text selection highlighting?

Answer:

According to Can I use, the

user-select

is currently supported in all browsers except Internet Explorer 9 and earlier versions (but sadly still needs a vendor prefix). All of the correct CSS variations are:

.noselect {
  -webkit-touch-callout: none; /* iOS Safari */
    -webkit-user-select: none; /* Safari */
     -khtml-user-select: none; /* Konqueror HTML */
       -moz-user-select: none; /* Old versions of Firefox */
        -ms-user-select: none; /* Internet Explorer/Edge */
            user-select: none; /* Non-prefixed version, currently
                                  supported by Chrome, Edge, Opera and Firefox */
}

<p>
  Selectable text.
</p>
<p class="noselect">
  Unselectable text.
</p>

Note that it’s a non-standard feature (i.e. not a part of any specification). It is not guaranteed to work everywhere, and there might be differences in implementation among browsers, and in the future browsers can drop support for it. More information can be found in Mozilla Developer Network documentation.

Alternative Answer:

In most browsers, this can be achieved using proprietary variations on the CSS

user-select

property, originally proposed and then abandoned in CSS 3 and now proposed in CSS UI Level 4:

*.unselectable {
   -moz-user-select: none;
   -khtml-user-select: none;
   -webkit-user-select: none;

   /*
     Introduced in Internet Explorer 10.
     See http://ie.microsoft.com/testdrive/HTML5/msUserSelect/
   */
   -ms-user-select: none;
   user-select: none;
}

For Internet Explorer < 10 and Opera < 15, you will need to use the

unselectable

attribute of the element you wish to be unselectable.

You can set this using an attribute in HTML:

<div id="foo" unselectable="on" class="unselectable">...</div>

Sadly this property isn’t inherited, meaning you have to put an attribute in the start tag of every element inside the

<div>

If this is a problem, you could instead use JavaScript to do this recursively for an element’s descendants:

function makeUnselectable(node) {
    if (node.nodeType == 1) {
        node.setAttribute("unselectable", "on");
    }
    var child = node.firstChild;
    while (child) {
        makeUnselectable(child);
        child = child.nextSibling;
    }
}

makeUnselectable(document.getElementById("foo"));

This tree traversal needs to be rerun whenever a new element is added to the tree, but it seems that it is possible to avoid this by adding a

mousedown

event handler that sets

unselectable

on the target of the event. See http://jsbin.com/yagekiji/1 for details. This still doesn’t cover all possibilities.

While it is impossible to initiate selections in unselectable elements, in some browsers (Internet Explorer and Firefox, for example) it’s still impossible to prevent selections that start before and end after the
unselectable element without making the whole document unselectable.

2. How to horizontally center a

<div>

within another

<div>

using CSS?

Answer: You can apply this CSS to the inner

<div>

#inner {
  width: 50%;
  margin: 0 auto;
}

Of course, you don’t have to set the

width

50%

. Any width less than the containing

<div>

will work. The

margin: 0 auto

is what does the actual centering.

If you are targeting Internet Explorer 8 (and later), it might be better to have this instead:

#inner {
  display: table;
  margin: 0 auto;
}

It will make the inner element center horizontally and it works without setting a specific

width

Working example here:

#inner {
  display: table;
  margin: 0 auto;
  border: 1px solid black;
}

#outer {
  border: 1px solid red;
  width:100%
}

<div id="outer">
  <div id="inner">Foo foo</div>
</div>

Alternative Answer:

If you don’t want to set a fixed width on the inner

div

you could do something like this:

#outer {
  width: 100%;
  text-align: center;
}

#inner {
  display: inline-block;
}

<div id="outer">  
    <div id="inner">Foo foo</div>
</div>

That makes the inner

div

into an inline element that can be centered with

text-align

3. How to change an HTML5 input’s placeholder color with CSS?

Answer:

There are three different implementations: pseudo-elements, pseudo-classes, and nothing.

WebKit, Blink (Safari, Google Chrome, Opera 15+) and Microsoft Edge are using a pseudo-element:
```
::-webkit-input-placeholder
```
. [Ref]
Mozilla Firefox 4 to 18 is using a pseudo-class:
```
:-moz-placeholder
```
(one colon). [Ref]
Mozilla Firefox 19+ is using a pseudo-element:
```
::-moz-placeholder
```
, but the old selector will still work for a while. [Ref]
Internet Explorer 10 and 11 are using a pseudo-class:
```
:-ms-input-placeholder
```
. [Ref]
April 2017: Most modern browsers support the simple pseudo-element
```
::placeholder
```
[Ref]

Internet Explorer 9 and lower does not support the

placeholder

attribute at all, while Opera 12 and lower do not support any CSS selector for placeholders. The discussion about the best implementation is still going on. Note the pseudo-elements act like real elements in the Shadow DOM.

padding

on an

input

will not get the same background color as the pseudo-element. CSS selectors User agents are required to ignore a rule with an unknown selector.

See Selectors Level 3:

a group of selectors containing an invalid selector is invalid.

So we need separate rules for each browser. Otherwise, the whole group would be ignored by all browsers.

::-webkit-input-placeholder { /* WebKit, Blink, Edge */
    color:    #909;
}
:-moz-placeholder { /* Mozilla Firefox 4 to 18 */
   color:    #909;
   opacity:  1;
}
::-moz-placeholder { /* Mozilla Firefox 19+ */
   color:    #909;
   opacity:  1;
}
:-ms-input-placeholder { /* Internet Explorer 10-11 */
   color:    #909;
}
::-ms-input-placeholder { /* Microsoft Edge */
   color:    #909;
}

::placeholder { /* Most modern browsers support this now. */
   color:    #909;
}

<input placeholder="Stack Snippets are awesome!">

Usage notes

Be careful to avoid bad contrasts. Firefox’s placeholder appears to be defaulting with reduced opacity, so need to use
```
opacity: 1
```
here.
Note that placeholder text is just cut off if it doesn’t fit – size your input elements in
```
em
```
and test them with big minimum font size settings. Don’t forget translations: some languages need more room for the same word.
Browsers with HTML support for
```
placeholder
```
but without CSS support for that (like Opera) should be tested too.
Some browsers use additional default CSS for some
```
input
```
types (
```
email
```
,
```
search
```
). These might affect the rendering in unexpected ways. Use the properties
```
-webkit-appearance
```
and
```
-moz-appearance
```
to change that. Example:

[type="search"] {
        -moz-appearance:    textfield;
        -webkit-appearance: textfield;
        appearance: textfield;
    }

Alternative Answer:

/* do not group these rules */
*::-webkit-input-placeholder {
    color: red;
}
*:-moz-placeholder {
    /* FF 4-18 */
    color: red;
    opacity: 1;
}
*::-moz-placeholder {
    /* FF 19+ */
    color: red;
    opacity: 1;
}
*:-ms-input-placeholder {
    /* IE 10+ */
    color: red;
}
*::-ms-input-placeholder {
    /* Microsoft Edge */
    color: red;
}
*::placeholder {
    /* modern browser */
    color: red;
}

<input placeholder="hello"/> <br />
<textarea placeholder="hello"></textarea>

This will style all

input

and

textarea

placeholders.

Important Note: Do not group these rules. Instead, make
a separate rule for every selector (one invalid selector in a group
makes the whole group invalid).

4. How to set cellpadding and cellspacing in CSS?

Answer:

For controlling “cellpadding” in CSS, you can simply use

padding

on table cells. E.g. for 10px of “cellpadding”:

td { 
    padding: 10px;
}

For “cellspacing”, you can apply the

border-spacing

CSS property to your table. E.g. for 10px of “cellspacing”:

table { 
    border-spacing: 10px;
    border-collapse: separate;
}

This property will even allow separate horizontal and vertical spacing, something you couldn’t do with old-school “cellspacing”.

Issues in IE <= 7

This will work in almost all popular browsers except for Internet Explorer up through Internet Explorer 7, where you’re almost out of luck. “Almost” because these browsers still support the

border-collapse

property, which merges the borders of adjoining table cells.

If you’re trying to eliminate cellspacing (that is,

cellspacing="0"

) then

border-collapse:collapse

should have the same effect: no space between table cells. This support is buggy, though, as it does not override an existing

cellspacing

HTML attribute on the table element.

In short: for non-Internet Explorer 5-7 browsers,

border-spacing

handles you. For Internet Explorer, if your situation is just right
(you want 0 cellspacing and your table doesn’t have it defined already),
you can use

border-collapse:collapse

table { 
    border-spacing: 0;
    border-collapse: collapse;
}

Note: For a great overview of CSS properties that one can apply to tables and for which browsers, see this fantastic Quirksmode page.

5. Is there a CSS parent selector?

Answer:

There is currently no way to select the parent of an element in CSS. If there was a way to do it, it would be in either of the current CSS selectors specs:

That said, the Selectors Level 4 Working Draft includes a

:has()

pseudo-class that will provide this capability. It will be similar to the jQuery implementation.

li:has(> a.active) { /* styles to apply to the li tag */ }

However, as of May 2020, this is still not supported by any browser. In the meantime, you’ll have to resort to JavaScript if you need to select a parent element.

Alternative Answer:

You can use this script:

*! > input[type=text] { background: #000; }

This will select any parent of text input. But wait, there’s still much more. If you want, you can select a specified parent:

.input-wrap! > input[type=text] { background: #000; }

Or select it when it’s active:

.input-wrap! > input[type=text]:focus { background: #000; }

Check out this HTML:

<div class="input-wrap">
    <input type="text" class="Name"/>
    <span class="help hide">Your name sir</span>
</div>

You can select that

span.help

when the

input

is active and show it:

.input-wrap! .help > input[type=text]:focus { display: block; }

There are many more capabilities; just check out the documentation of the plugin. By the way, it works in Internet Explorer.

6. How to disable the resizable property of a textarea?

Answer:

The following CSS rule disables resizing behavior for

textarea

elements:

textarea {
  resize: none;
}

To disable it for some (but not all)

textarea

s, there are a couple of options. To disable a specific

textarea

with the

name

attribute set to

foo

(i.e.,

<textarea name="foo"></textarea>

textarea[name=foo] {
  resize: none;
}

Or, using an

id

attribute (i.e.,

<textarea id="foo"></textarea>

#foo {
  resize: none;
}

The W3C page lists possible values for resizing restrictions: none, both, horizontal, vertical, and inherit:

textarea {
  resize: vertical; /* user can resize vertically, but width is fixed */
}

Review a decent compatibility page to see what browsers currently support this feature. The dimensions can be further restrained in CSS using max-width, max-height, min-width, and min-height.

Note: This property does nothing unless the overflow property is something other than visible, which is the default for most elements.

So generally to use this, you’ll have to set something like overflow: scroll; http://css-tricks.com/almanac/properties/r/resize/

Alternative Answer:

In CSS

textarea {
    resize: none;
}

7. Is it possible to have a list without bullets?

Answer:

You can remove bullets by setting the

list-style-type

none

on the CSS for the parent element (typically a

<ul>

), for example:

ul {
  list-style-type: none;
}

You might also want to add

padding: 0

and

margin: 0

to that if you want to remove indentation as well. See Listutorial for a great walkthrough of list formatting techniques.

Alternative Answer:

If you’re using Bootstrap, it has an “unstyled” class:

Remove the default list-style and left padding on list items (immediate children only).

Bootstrap 2:

<ul class="unstyled">
   <li>...</li>
</ul>

http://twitter.github.io/bootstrap/base-css.html#typography

Bootstrap 3 and 4:

<ul class="list-unstyled">
   <li>...</li>
</ul>

Bootstrap 3: http://getbootstrap.com/css/#type-lists

Bootstrap 4: https://getbootstrap.com/docs/4.3/content/typography/#unstyled

8. How to give text or an image a transparent background using CSS?

Answer:

You can either use a semi-transparent PNG image or use CSS 3:

background-color: rgba(255, 0, 0, 0.5);

Here’s an article from css3.info, Opacity, RGBA and compromise.

<p style="background-color: rgba(255, 0, 0, 0.5);">
  <span>Hello, World!</span>
</p>

Alternative Answer:

In Firefox 3 and Safari 3, you can use RGBA. A little known trick is that you can use it in Internet Explorer as well using the gradient filter.

background-color: rgba(0, 255, 0, 0.5);
filter: progid:DXImageTransform.Microsoft.Gradient(GradientType=0, StartColorStr='#7F00FF00', EndColorStr='#7F00FF00');

The first hex number defines the alpha value of the color.

Full solution for all browsers:

.alpha60 {
    /* Fallback for web browsers that doesn't support RGBa */
    background: rgb(0, 0, 0) transparent;
    /* RGBa with 0.6 opacity */
    background: rgba(0, 0, 0, 0.6);
    /* For IE 5.5 - 7*/
    filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#99000000);
    /* For IE 8*/
    -ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#99000000)";
}

This is from CSS background transparency without affecting child elements, through RGBa and filters.

This is when using the following code:

 <head>
     <meta http-equiv="X-UA-Compatible" content="IE=edge" >
    <title>An XHTML 1.0 Strict standard template</title>
     <meta http-equiv="content-type" content="text/html;charset=utf-8" />
    <style type="text/css" media="all">
         .transparent-background-with-text-and-images-on-top {
             background: rgb(0, 0, 0) transparent;   /* Fallback for web browsers that doesn't support RGBa */
            background: rgba(0, 0, 0, 0.6);   /* RGBa with 0.6 opacity */
             filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#99000000);  /* For IE 5.5 - 7*/
            -ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#99000000, endColorstr=#99000000)";  /* For IE 8*/
         }
     </style>
 </head>

 <body>
     <div class="transparent-background-with-text-and-images-on-top">
         <p>Here some content (text AND images) "on top of the transparent background"</p>
        <img src="http://i.imgur.com/LnnghmF.gif">
     </div>
 </body>
 </html>

9. How to vertically center text with CSS?

Answer:

You can try this basic approach:

div {
  height: 100px;
  line-height: 100px;
  text-align: center;
  border: 2px dashed #f69c55;
}

<div>
  Hello World!
</div>

It only works for a single line of text though, because we set the line’s height to the same height as the containing box element.

A more versatile approach

This is another way to align text vertically. This solution will work for a single line and multiple lines of text, but it still requires a fixed height container:

div {
  height: 100px;
  line-height: 100px;
  text-align: center;
  border: 2px dashed #f69c55;
}
span {
  display: inline-block;
  vertical-align: middle;
  line-height: normal;
}

<div>
  <span>Hello World!</span>
</div>

The CSS just sizes the

<div>

, vertically center aligns the

<span>

by setting the

<div>

‘s line-height equal to its height, and makes the

<span>

an inline-block with

vertical-align: middle

. Then it sets the line-height back to normal for the

<span>

, so its contents will flow naturally inside the block.

Simulating table display

And here is another option, which may not work on older browsers that don’t support display: table and

display: table-cell

(basically just Internet Explorer 7).

Using CSS we simulate table behavior (since tables support vertical alignment), and the HTML is the same as the second example:

div {
  display: table;
  height: 100px;
  width: 100%;
  text-align: center;
  border: 2px dashed #f69c55;
}
span {
  display: table-cell;
  vertical-align: middle;
}

<div>
  <span>Hello World!</span>
</div>

Using absolute positioning

This technique uses an absolutely positioned element setting top, bottom, left, and right to 0. It is described in more detail in an article in Smashing Magazine, Absolute Horizontal And Vertical Centering In CSS.

div {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100px;
  width: 100%;
  border: 2px dashed #f69c55;
}

<div>
  <span>Hello World!</span>
</div>

Alternative Answer:

Another way is with Flexbox. Just add the following code to the container element:

display: flex;
justify-content: center; /* align horizontal */
align-items: center; /* align vertical */

Flexbox demo 1

.box {
  height: 150px;
  width: 400px;
  background: #000;
  font-size: 24px;
  font-style: oblique;
  color: #FFF;
  text-align: center;
  padding: 0 20px;
  margin: 20px;
  display: flex;
  justify-content: center;
  /* align horizontal */
  align-items: center;
  /* align vertical */
}

<div class="box">
  Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh
</div>

Alternatively, instead of aligning the content via the container, the flexbox can also center the flex item with an auto margin when there is only one flex-item in the flex container. So to center the flex item both horizontally and vertically just set it with

margin:auto

Flexbox Demo 2

.box {
  height: 150px;
  width: 400px;
  background: #000;
  font-size: 24px;
  font-style: oblique;
  color: #FFF;
  text-align: center;
  padding: 0 20px;
  margin: 20px;
  display: flex;
}
.box span {
  margin: auto;
}

<div class="box">
  <span>margin:auto on a flex item centers it both horizontally and vertically</span> 
</div>

All the above applies to centering items while laying them out in horizontal rows. This is also the default behavior because by default the value for

flex-direction

row

If, however, flex-items need to be laid out in vertical columns, then

flex-direction: column

should be set on the container to set the main-axis as column and additionally the

justify-content

and

align-items

properties now work the other way around with

justify-content: center

centering vertically and

align-items: center

centering horizontally)

flex-direction: column

demo

.box {
  height: 150px;
  width: 400px;
  background: #000;
  font-size: 18px;
  font-style: oblique;
  color: #FFF;
  display: flex;
  flex-direction: column;
  justify-content: center;
  /* vertically aligns items */
  align-items: center;
  /* horizontally aligns items */
}
p {
  margin: 5px;
  }

<div class="box">
  <p>
    When flex-direction is column...
  </p>
  <p>
    "justify-content: center" - vertically aligns
  </p>
  <p>
    "align-items: center" - horizontally aligns
  </p>
</div>

A good place to start with Flexbox to see some of its features and get syntax for maximum browser support is flexyboxes Also, browser support nowadays is very good: caniuse

For cross-browser compatibility for

display: flex

and

align-items

, you can use the following:

display: -webkit-box;
display: -webkit-flex;
display: -moz-box;
display: -ms-flexbox;
display: flex;
-webkit-flex-align: center;
-ms-flex-align: center;
-webkit-align-items: center;
align-items: center;

10. How to transition height: 0; to height: auto; using CSS?

Answer:

Use

max-height

in the transition and not

height

. And set a value on

max-height

to something bigger than your box will ever get.

See JSFiddle demo.

#menu #list {
    max-height: 0;
    transition: max-height 0.15s ease-out;
    overflow: hidden;
    background: #d5d5d5;
}

#menu:hover #list {
    max-height: 500px;
    transition: max-height 0.25s ease-in;
}

<div id="menu">
    <a>hover me</a>
    <ul id="list">
        <!-- Create a bunch, or not a bunch, of li's to see the timing. -->
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
    </ul>
</div>

11. How to use transitions on the CSS display property?

Answer:

You can concatenate two transitions or more and

visibility

is what comes handy this time.

div {
  border: 1px solid #eee;
}
div > ul {
  visibility: hidden;
  opacity: 0;
  transition: visibility 0s, opacity 0.5s linear;
}
div:hover > ul {
  visibility: visible;
  opacity: 1;
}

<div>
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
  </ul>
</div>

(Don’t forget the vendor prefixes to the

transition

property.)

Alternative Answer:

You need to hide the element by other means in order to get this to work. You can accomplish the effect by positioning both

<div>

s absolutely and setting the hidden one to

opacity: 0

. If you even toggle the

display

property from

none

block

, your transition on other elements will not occur.

To work around this, always allow the element to be

display: block

, but hide the element by adjusting any of these means:

Set the
```
height
```
to
```
0
```
.
Set the
```
opacity
```
to
```
0
```
.
Position the element outside of the frame of another element that has
```
overflow: hidden
```
.

There are likely more solutions, but you cannot perform a transition if you toggle the element to

display: none

For example, you may attempt to try something like this:

div {
    display: none;
    transition: opacity 1s ease-out;
    opacity: 0;
}
div.active {
    opacity: 1;
    display: block;
}

But that will not work. Because of this, you will always need to keep the element

display: block

, but you could get around it by doing something like this:

div {
    transition: opacity 1s ease-out;
    opacity: 0;
    height: 0;
    overflow: hidden;
}
div.active {
    opacity: 1;
    height: auto;
}

In Conclusion

These are the 11 most commonly asked questions on CSS. If you have any
suggestions or any confusion, please comment below. If you need any
help, we will be glad to help you.

Hope this article helped you.

This post was originally posted on DevPost by Truemark.

Previously published at https://thedevpost.com/blog/11-most-asked-questions-on-css/