Adjacent inline elements not separated by whitespace will bump up against each other when viewed in an unstyled manner, which usually isn't desirable.
Examples of incorrect code for this rule:
<div><a></a><a></a></div>
<div><a></a><span></span></div>
Inferno.createElement("div", undefined, [Inferno.createElement("a"), Inferno.createElement("span")]);
Examples of correct code for this rule:
<div><div></div><div></div></div>
<div><a></a> <a></a></div>
Inferno.createElement("div", undefined, [Inferno.createElement("a"), " ", Inferno.createElement("a")]);