Bug 1500389 - Set margin to 0 instead of initial for <p>;r=ladybenko,daisuke

Depends on D17917
As discussed on slack. Let me know if you'd prefer another comment and/or moving this to the message CSS file?

Differential Revision: https://phabricator.services.mozilla.com/D17918

--HG--
extra : moz-landing-system : lando

devtools/client/aboutdebugging-new/src/base.css

@@ -99,8 +99,8 @@ a:active {
 }
 
 p {
-  /* reset for the default <p> styles */
-  margin: initial;
+  /* remove default browser margins on <p> elements */
+  margin: 0;
 }