The transition-timing-function property specifies the speed curve of the transition effect, allowing for the effect to change speed over its duration.

The following will animate the width, on hover, after a delay of 250 milliseconds, over the course of 0.4 seconds, using ease-in-out.

div {
    width: 50%;
    transition-property: width;
    transition-duration: 0.4s;
    transition-timing-function: ease-in-out;
    transition-delay: 250ms;
}
div:hover {
    width: 100%;
}

Default value	ease
Inherited values	no
Can it be animated?	no

These are the allowed values.

Value	Description
ease	Specifies a transition effect with a slow start, then fast, then end slowly (equivalent to cubic-bezier(0.25,0.1,0.25,1)) (default)
linear	Specifies a transition effect with the same speed from start to end (equivalent to cubic-bezier(0,0,1,1))
ease-in	Specifies a transition effect with a slow start (equivalent to cubic-bezier(0.42,0,1,1))
ease-out	Specifies a transition effect with a slow end (equivalent to cubic-bezier(0,0,0.58,1))
ease-in-out	Specifies a transition effect with a slow start and end (equivalent to cubic-bezier(0.42,0,0.58,1))
step-start	Equivalent to steps(1, start)
step-end	Equivalent to steps(1, end)
steps(int,start|end)	Specifies a stepping function, with two parameters; the first parameter specifies the number of intervals in the function (a positive integer greater than 0); the second parameter, which is optional, is either the value "start" or "end", and specifies the point at which the change of values occur within the interval (if omitted, it is given the value "end")
cubic-bezier(n,n,n,n)	Define your own values in the cubic-bezier function; possible values are numeric values from 0 to 1
initial	Sets this property to its default value
inherit	Inherits this property from its parent element

The HTML element can also be styled using JavaScript and the element’s id.

document.getElementById('my_div').style.transitionTimingFunction = 'ease-in-out';

<button onclick='my_function()'>Click Here</button>

<script>
function my_function() {
    document.getElementById('my_div').style.transitionTimingFunction = 'ease-in-out';
}
</script>

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