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How can I add interactive semantics to a pseudo element with CSS or JavaScript?

I have a small carousel that plays automatically on page load, using HTML, CSS and JavaScript and definitely no jQuery.
To add a pause/play option there is a span with role="checkbox" followed by a label.
The label itself is hidden and has no content. The span has two pseudo elements. On first showing, the pseudo element shows the ⏸ character, controlled by a CSS ::after class. When clicked, the span has the class "is-clicked" added, at which point the ▶ character is displayed, controlled by another ::after class
It is focusable and can be activated with the keyboard by hitting the Enter key, but when I check with Lighthouse, I keep getting the "Focusable elements should have interactive semantics".
Why is this?
Here is the code:
/* detect keyboard users */
function handleFirstTab(e) {
if (e.key === 'Tab') { // the 'I am a keyboard user' key
document.body.classList.add('user-is-tabbing');
window.removeEventListener('keydown', handleFirstTab);
}
}
let checkboxEl = document.getElementById('checkbox');
let labelEl = document.getElementById('checkboxLabel');
labelEl.onclick = function handleLabelClick() {
checkboxEl.focus();
toggleCheckbox();
}
function toggleCheckbox() {
let isChecked = checkboxEl.classList.contains('is-checked');
checkboxEl.classList.toggle('is-checked', !isChecked);
checkboxEl.setAttribute('aria-checked', !isChecked);
}
checkboxEl.onclick = function handleClick() {
toggleCheckbox();
}
checkboxEl.onkeypress = function handleKeyPress(event) {
let isEnterOrSpace = event.keyCode === 32 || event.keyCode === 13;
if(isEnterOrSpace) {
toggleCheckbox();
}
}
.link {
height: auto;
border: 1px solid #000;
margin-bottom: 1rem;
width: 80%;
display: block;
}
#carousel-checkbox {
margin-bottom: 1rem;
height: 50px;
width: 100px;
display: inline-block;
}
#carousel-checkbox input {
display: none;
}
#carousel-checkbox label {
display: inline-block;
vertical-align: middle;
}
#carousel-checkbox #checkbox {
position: relative;
top: 0;
left: 30px;
padding: 0.5rem 1rem;
background: rgba(255,255,255, 0.5);
}
#carousel-checkbox #checkbox:hover {
cursor: pointer;
}
#carousel-checkbox #checkbox:focus {
border: 1px dotted var(--medium-grey);
}
#carousel-checkbox #checkbox::after {
content: "⏸";
font-size: 1.5rem;
color: var(--theme-dark);
}
#carousel-checkbox #checkbox.is-checked::after {
content: "▶";
}
<div class="link">A bit of text with a dummy link to demonstrate the keyboard tabbing navigation. </div>
<div id="carousel-checkbox"><span id="checkbox" tabindex="0" role="checkbox" aria-checked="false" aria-labelledby="checkboxLabel"></span><label id="checkboxLabel"></label></div>
<div class="link">Another link to another dummy link</div>
Why is this? Is it because the pseudo elements don't have a name attribute or something like that?
I have tried a different way, by dropping the pseudo elements and trying to change the span innerHTML depending on whether the class 'is-clicked' exists or not, but although I can get the pause character to display initially, it won't change the innerHTML to the play character when the span is clicked again.

Short Answer
This is a warning rather than an error, it is telling you to check that the item actually is interactive.
Now you have got the interactivity on the element so you can ignore that issue.
Long answer
Why not just use a <input type="checkbox"> and save yourself an awful lot of extra work?
You can hide a checkbox with a visually hidden class.
This then allows you to do the same trick with a pseudo element as the visual representation of the state.
I have made several changes to your example that mean you don't have to worry about capturing keypresses etc. and can just use a click handler so your JS is far simpler.
Notice the trick with the label where I add some visually hidden text within it so the label is still visible (so we can still use psuedo elements!).
I then use #checkbox1 ~ label to access the label with CSS so we can change the state.
The final thing to notice is how I changed the content property slightly. This is because some screen readers will try and read out pseudo elements so I added alt text that was blank. Support isn't great at just over 70%, but it is worth adding for browsers that do support it.
Example
The below hopefully illustrates a way of achieving what you want with a checkbox.
There may be a few errors as I just adapted your code so please do not just copy and paste!
note: a checkbox should not work with Enter, only with Space. If you want it to work with both it should instead be a toggle switch etc. so that would be a completely different pattern.
let checkboxEl = document.getElementById('checkbox1');
let labelEl = document.querySelector('#checkboxLabel');
function toggleCheckbox() {
let isChecked = checkboxEl.classList.contains('is-checked');
checkboxEl.classList.toggle('is-checked', !isChecked);
checkboxEl.setAttribute('aria-checked', !isChecked);
}
checkboxEl.onclick = function handleClick() {
toggleCheckbox();
}
.link {
height: auto;
border: 1px solid #000;
margin-bottom: 1rem;
width: 80%;
display: block;
}
#carousel-checkbox {
margin-bottom: 1rem;
height: 50px;
width: 100px;
display: inline-block;
}
.visually-hidden {
border: 0;
padding: 0;
margin: 0;
position: absolute !important;
height: 1px;
width: 1px;
overflow: hidden;
clip: rect(1px 1px 1px 1px); /* IE6, IE7 - a 0 height clip, off to the bottom right of the visible 1px box */
clip: rect(1px, 1px, 1px, 1px); /*maybe deprecated but we need to support legacy browsers */
clip-path: inset(50%); /*modern browsers, clip-path works inwards from each corner*/
white-space: nowrap; /* added line to stop words getting smushed together (as they go onto seperate lines and some screen readers do not understand line feeds as a space */
}
#carousel-checkbox label {
display: inline-block;
vertical-align: middle;
}
#carousel-checkbox #checkbox1 {
position: relative;
top: 0;
left: 30px;
padding: 0.5rem 1rem;
background: rgba(255,255,255, 0.5);
}
#carousel-checkbox #checkbox1 ~label:hover {
cursor: pointer;
}
#carousel-checkbox #checkbox1:focus ~ label {
border: 1px dotted #333;
}
#carousel-checkbox #checkbox1 ~label::after {
content: "⏸" / "";
font-size: 1.5rem;
color: #000;
}
#carousel-checkbox #checkbox1.is-checked ~label::after {
content: "▶" / "";
}
<div class="link">A bit of text with a dummy link to demonstrate the keyboard tabbing navigation. </div>
<div id="carousel-checkbox">
<input type="checkbox" id="checkbox1" class="visually-hidden">
<label for="checkbox1" id="checkboxLabel">
<span class="visually-hidden">Pause animations</span>
</label>
</div>
<div class="link">Another link to another dummy link</div>

In the end, I gave up on using a checkbox, due to the difficulties with iPad/iOS not responding to checkbox events. Whilst it worked in codepen on iOS it wouldn't work on the actual site. So I switched to a button.
Here is the code, which is fully accessible with no 'interactive semantics' warnings, shown with some dummy slides. The animation is based on having only three slides. If you wanted more or less, then the timings would have to be adjusted accordingly. All I need now is to style the pause button.
let element = document.getElementById("pause");
function toggleButton() {
element.classList.toggle("paused");
if (element.innerHTML === "⏸") {
element.innerHTML = "▶";
}
else {
element.innerHTML = "⏸";
}
}
element.onclick = function handleClick() {
toggleButton();
}
#carousel {
height: auto;
max-width: 1040px;
position: relative;
margin: 4rem auto 0;
}
#carousel > * {
animation: 12s autoplay6 infinite linear;
position: absolute;
top: 0;
left: 0;
opacity: 0.0;
}
#carousel .one {
position: relative;
}
.homeSlides {
height: 150px;
width: 400px;
background-color: #ff0000;
}
.homeSlides.two {
background-color: #0fff00;
}
.homeSlides.three {
background-color: #e7e7e7;
}
#keyframes autoplay6 {
0% {opacity: 0.0}
4% {opacity: 1.0}
33.33% {opacity: 1.0}
37.33% {opacity: 0.0}
100% {opacity: 0.0}
}
#carousel > *:nth-child(1) {
animation-delay: 0s;
}
#carousel > *:nth-child(2) {
animation-delay: 4s;
}
#carousel > *:nth-child(3) {
animation-delay: 8s;
}
#carousel-button {
position: relative;
height: 100%;
width: auto;
}
#carousel-button button {
position: absolute;
top: -3.5rem;
right: 5rem;
padding: 0 0.5rem 0.25rem;;
background: #fff;
z-index: 98;
font-size: 2rem;
cursor: pointer;
}
body.user-is-tabbing #carousel-button button:focus {
outline: 1px dotted #333;
}
body:not(.user-is-tabbing) #carousel-button button:focus {
outline: none;
}
#carousel-button button:hover {
cursor: pointer;
}
#carousel-button ~ #carousel * {
animation-play-state: running;
}
#carousel-button button.paused ~ #carousel * {
animation-play-state: paused;
}
<div id="carousel-button"><button id="pause" class="">⏸</button>
<div id="carousel">
<div class="homeSlides one">This is div one</div>
<div class="homeSlides two">This is div two</div>
<div class="homeSlides three">This is div three</div>
</div>
</div>

Scrolling Popup Window Out of Screen

We open popup window. We want to slide this off the screen. The "Right" and "Top" parts can be made as much as possible. How can we slide out of the screen?
var wind = window.open("https://www.google.com","","width=50,height=50,left=1500%,top=1000%");

While I'm not exactly sure about your particular use case, but by toggling opacity and right/left, you can achieve the "effect" of the div going out of the screen.
function move() {
let element = document.getElementById("box");
element.style.transform = "translate(-400px)";
element.style.right = 0;
element.style.opacity = 0;
}
#box {
width: 300px;
height: 200px;
background: blue;
margin-bottom: 20px;
color: white;
transition: all 0.3s ease;
opacity: 1;
position: absolute;
}
#btn {
margin-top: 300px;
}
<div id="box">Hello</div>
<button id="btn" onclick="move()">Click</button>

Responsive YouTube embed not displaying in Firefox

This embed method appears to be working on all browsers except Firefox; I signed up for a free trial at crossbrowsertesting.com to check. I’m not doing a direct iFrame embed, and all the questions and answers I’ve found relate to that. I’m using this method: A Better Method for Embedding YouTube Videos on your Website. This method:
embeds the thumbnail image of a YouTube video and the actual video player is loaded only when the user manually clicks the thumbnail
The closest issue I could find on Stack Overflow was YouTube embed not working in Firefox. But this does not apply.
Here are screenshots of it displaying properly in Chrome:
And not displaying in Firefox:
In the Firefox image you can see the margin showing up in the inspector as I hover that <div>.
When I set an explicit height value this thumbnail does show up in Firefox, but it negates the responsiveness of the method.
document.addEventListener("DOMContentLoaded",
function() {
var div, n,
v = document.getElementsByClassName("youtube-player");
for (n = 0; n < v.length; n++) {
div = document.createElement("div");
div.setAttribute("data-id", v[n].dataset.id);
div.innerHTML = labnolThumb(v[n].dataset.id);
div.onclick = labnolIframe;
v[n].appendChild(div);
}
});
function labnolThumb(id) {
var thumb = '<img src="https://i.ytimg.com/vi/ID/hqdefault.jpg">',
play = '<div class="play"></div>';
return thumb.replace("ID", id) + play;
}
function labnolIframe() {
var iframe = document.createElement("iframe");
var embed = "https://www.youtube.com/embed/ID?autoplay=1";
iframe.setAttribute("src", embed.replace("ID", this.dataset.id));
iframe.setAttribute("frameborder", "0");
iframe.setAttribute("allowfullscreen", "1");
this.parentNode.replaceChild(iframe, this);
}
.youtube-player {
position: relative;
padding-bottom: 56.23%;
/* Use 75% for 4:3 videos */
height: 0;
overflow: hidden;
max-width: 100%;
background: #000;
margin: 5px;
}
.youtube-player iframe {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
z-index: 100;
background: transparent;
}
.youtube-player img {
bottom: 0;
display: block;
left: 0;
margin: auto;
max-width: 100%;
width: 100%;
position: absolute;
right: 0;
top: 0;
border: none;
height: auto;
cursor: pointer;
-webkit-transition: .4s all;
-moz-transition: .4s all;
transition: .4s all;
}
.youtube-player img:hover {
-webkit-filter: brightness(75%);
}
.youtube-player .play {
height: 72px;
width: 72px;
left: 50%;
top: 50%;
margin-left: -36px;
margin-top: -36px;
position: absolute;
background: url("//i.imgur.com/TxzC70f.png") no-repeat;
cursor: pointer;
}
<div class="youtube-player" data-id="VIDEO_ID"></div>

I solved this by removing redundant flexbox classes from the parent div. Specifically:
frow direction-column
(these are from the nice flexbox grid framework FrowCSS)
I don't fully understand why, but these must have been interfering with the requried styles in FireFox. Glad I figured it out, I had been wrestling with this 2-3 hours before I posted on SO. Hope this helps someone else in future.

I wrapped a div around the youtube-player and set that to display block and added a width of 100%. That solved the problem. Adding those to the youtube-player itself didn't work though.

What's causing the paint event in this CSS animation?

I'm building an animated menu based off of Smooth as Butter: Achieving 60 FPS Animations with CSS3.
The canonical demo which I'm comparing my code against is: http://codepen.io/Onyros/pen/jAJxkW
This is my demo code:
var navLayer = document.querySelector('.nav-layer'),
open = document.querySelector('.open'),
close = document.querySelector('.close');
function toggleNav() {
navLayer.classList.add('nav-layer__animating');
if (navLayer.classList.contains('nav-layer__visible')) {
navLayer.classList.remove('nav-layer__visible');
} else {
navLayer.classList.add('nav-layer__visible');
}
}
open.addEventListener('click', toggleNav, false);
close.addEventListener('click', toggleNav, false);
navLayer.addEventListener('transitionend', function() {
navLayer.classList.remove('nav-layer__animating');
}, false);
.nav-layer {
position: fixed;
top: 0;
left: 0;
height: 100vh;
width: 100vw;
z-index: 2;
pointer-events: none;
}
.nav-layer__visible {
pointer-events: auto;
}
.header {
position: fixed;
top: 0;
left: 0;
background-color: beige;
height: 10vh;
width: 100vw;
z-index: 1;
}
.nav {
position: relative;
height: 100vh;
width: 90vw;
background-color: aquamarine;
z-index: 3;
transform: translateX(-91vw);
will-change: transform;
}
.nav-layer__animating .nav {
transition: all 300ms ease-in;
}
.nav-layer__visible.nav-layer__animating .nav {
transition: all 300ms ease-out;
}
.nav-layer__visible .nav {
transform: none;
}
<div class="nav-layer">
<nav class="nav">
<button class="close">Close</button>
</nav>
</div>
<header class="header"><button class="open">Menu</button></header>
<article>
<p>O hai</p>
</article>
When I run the canonical demo with paint flashing enabled in Chrome DevTools, I don't see any paint events.
When I run my demo, I see a flash of paint when I close the menu.
The event log in DevTools shows that there was a paint on #document and another on nav.nav.
The answer I'm looking for here is: find the CSS property in the canonical demo which is preventing this flash of paint. Or, maybe the difference is in how I've structured my HTML. I think the JS is equivalent, so it's unlikely to be there.
Please provide your methodology, too!

Moving the whole page to left and then right

i am looking for this kind of template . Moving the page to left and then page to right. Can anyone tell me how can i make this or is there any javascript example similar to this.

Create two <div>s, put them next to each other, make them take up the whole window, and change them as needed.
HTML:
<div class="left">left</div>
<div class="right">right</div>
CSS:
body {
margin: 0;
}
.left {
background-color: green;
bottom: 0;
left: 0;
position: fixed;
top: 0;
transition: width 1s;
width: 0;
}
.left.active {
width: 200px;
}
.right {
background-color: red;
bottom: 0;
left: 0;
position: fixed;
right: 0;
top: 0;
transition: left 1s;
}
.right.active {
left: 200px;
}
JS (width jQuery):
$('.right').on('click', function() {
$('.left').toggleClass('active');
$('.right').toggleClass('active');
});
And here's a fiddle.

Using .toggle(effect,options,duration) method to moving the page to left to right.
// Set the effect type
var effect = 'slide';
// Set the options for the effect type chosen
var options = { direction: 'right' };
// Set the duration (default: 400 milliseconds)
var duration = 700;
$('#Id').toggle(effect, options, duration);
Taken via this link

If you want it to animate smooth on all devices you should use css transitions and transforms. Hiding and showing would be as basic as toggling a class then.
The example in jsfiddle
<style media="screen">
.wrapper {
width: 100%;
overflow: hidden;
}
.menu {
height: 100vh;
width: 100px;
background: #ABC;
color: white;
position: absolute;
left:0;
transition: transform 0.3s;
transform: translateX(-100px);
}
.content {
transition: transform 0.3s;
}
.active .menu {
transform: translateX(0);
}
.active .content {
transform: translateX(100px);
}
</style>
<button class="toggle">Toggle</button>
<div class="wrapper">
<div class="menu">
My menu
</div>
<div class="content">
My content
</div>
</div>
<script type="text/javascript">
document.querySelector('.toggle').addEventListener('click', function(event) {
event.preventDefault();
document.querySelector('.wrapper').classList.toggle("active");
});
</script>
NB! Supported from IE10. IE 9 will support without the animation and you probably should add the needed -ms-, -webkit-, -moz-, etc prefixes to support the older browsers if needed for transition and transform properties.
Also I advise not animating body or html with this method and put the content of page in the wrapper (in .content in the examples case). Moving body and html directly may lead to unpleasant surprises later.