Effect of highlighting text with pen

This codepen is inspired by work by Sten Hougaard

Preparations

Let's create two containers for different implementations of animation:

<div class="container">
 <h1>Animated text
  <svg viewbox="0 0 235 40" xmlns="http://www.w3.org/2000/svg" id="animated-svg">
   <path d="..."/>
  </svg>
 </h1>
</div>

<div class="container">
 <h1 id="clickable-header">Click on me!
  <svg viewbox="0 0 235 40" xmlns="http://www.w3.org/2000/svg" id="clickable-svg">
   <path d="..."/>
  </svg>
 </h1>
</div>

I've created svg using Adobe illustrator, the main feature for it must be small height and big width.
It's important to mention that svg is a child of a header element since we will position svg relative to it.

CSS

We transform our containers to flexboxes to gracefully position everything in the center.

.container {
 display: flex;
 place-content: center;
}

h1 {
 display: inline;
 position: relative;
}   

inline is used to simply reduce width of the element to its content since svg will rely on it

Next, we should position svg relative to a header:

svg {
 min-width: 110%;
 min-height: 100%;
 position: absolute;
 top: 50%;
 left: 50%;
 transform: translate(-50%,-50%);
}

We use standard method to position relative element in the center of the parent. To adjust svg to the text, we should apply min-width and min-height.

Animation logic with javascript

To animate elements in javascript we can use wonderful animate(keyframes, options) method.

Keyframes

Let's dissect the getDrawingParameters function, that gives properties to iterate over:

const getDrawingParameters = (path) => {
 const length = path.getTotalLength();
 path.style.strokeDasharray = length;
 const drawingProperties = [
  { strokeDashoffset: length, easing: "ease-in" },
  { strokeDashoffset: 0, offset: 0.15 }
 ];
 return drawingProperties;
};

First, we need to know how long our path actually is, by using svg method getTotalLength:

const length = path.getTotalLength();

Now, we should use our computed length to simulate drawing. Let's define our starting point. We need attribute strokeDasharray:

path.style.strokeDasharray = length;

Here, we tell this attribute to draw path with alternation of one dash (size=length) and one gap (size=length).

The next attribute, where we need length is stroke-dashoffset:

path.style.strokeDashoffset = length;

This value tells that the dash array computation is pulled by length value. And since, we set our dash array to 'dash-length gap-length dash-length gap-length ...', the starting path will be empty (gap).

So, we define our iterable properties in array:

 const drawingProperties = [
  { strokeDashoffset: length, easing: "ease-in" },
  { strokeDashoffset: 0, offset: 0.15 }
 ];

We use offset to define relative time at which this property must be reached. It was made to imitate delay at each iteration.

Options

The second argument of the animate method is options:

animatedPath.animate(
 getDrawingParameters(animatedPath), 
 {duration: 10000, iterations: Infinity}
);

We set our animation to infinite iterations and 10 seconds duration. Remember offset? our "active" animation takes only 1.5 seconds.