This uses CSS variables to specify a dynamic blink rate. Check browser compatability here.
View the source on Github.
This will blink
<p class="blinking">This will blink</p>
This will blink
<p class="blinking" style="--rate: 2s">This will blink</p>
<h2>12<span class="blinking" style="--opacity: 0.4">:</span>00</h2>