How can I modify html from a widget?

Hello,
I want to modify the html code from a widget that is shown to users visiting the website.
For example:

<div class="widget"><h3>List of Categories</h3><ul>	<li><a href="https://www.domain.com/category1/">Category 1</a>
</li>
	<li><a href="https://www.domain.com/category2/">Category 2</a>
</li></ul></div>

I want to remove the spaces of code. Change for:

<div class="widget"><h3>List of Categories</h3><ul><li><a href="https://www.domain.com/category1/">Category 1</a></li><li><a href="https://www.domain.com/category2/">Category 2</a></li></ul></div>

Someone know how I can do this? I want to do it because one of my clients is too much perfectionist and he doesn’t want spaces in html code.

Thx
Caio