How to set only vertical padding?

If I understand what you want : you're looking for a way to only assign vertical padding with padding property (and keep original horizontal padding if it's set).

So you already have the answer :

.myElement { padding-top: 20px; padding-bottom: 20px;  }

You can't do that only with padding property, or you'll need to set horizontal value.

Or, you can consider using CSS preprocessors (such as Sass or Less), it will surely helps you to achieve this.

#ul1, #ul2{
  padding:20px 0; /* top and bottom 20px, left and right 0 */
}

http://www.w3schools.com/css/css_padding.asp

you can only set padding to either a fixed length or a percent, no other values I'm afraid

These days you can do it like this:

.myElement { padding-block: 20px; }

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