How to remove the blue highlight of button on mobile?

You can add:

-webkit-tap-highlight-color: transparent;

You can also add this to your stylesheets to define it globally:

input,
textarea,
button,
select,
a {
  -webkit-tap-highlight-color: rgba(0,0,0,0);
}

Hope this helps :)

You can find the documentation here for more info: https://developer.apple.com/library/archive/documentation/AppleApplications/Reference/SafariWebContent/AdjustingtheTextSize/AdjustingtheTextSize.html#//apple_ref/doc/uid/TP40006510-SW5

* {
    -webkit-tap-highlight-color: transparent;
}

Test it.

There will be no highlighting (at least in Chrome 88+) at all if you remove cursor: pointer from #add selector. And if you need it in the "desktop" mode, use something like this:

@media (min-width: 768px) {
    #add {
        cursor: pointer;
    }
}

You just need to add style="-webkit-tap-highlight-color: transparent;"

-webkit-tap-highlight-color is a non-standard feature (mdn). It won't work in browser like safari 14.

Instead, you can use

{ outline: none; }

or apply it specifically through selector

a:focus,a:visited,a:active{
  outline: none;
}

Try it

add to the button style "cursor: default;"

This will make a cursor:pointer; it turns into a "default", but will remove the blue shadow on the buttons on the mobile screen as you want

In addition to all the answers here, you have to also specify the background css property explicitly yourself to the button.