Comment by thybag

<script>x=document;v='innerHTML';o='onclick';x.body[v]="<input id=a><b id=b>X</b><p id=d>clear<ul id=c>",b[o]=e=>{let b=x.createElement("li");c.append(b);b[v]=a.value,a.value="",b[o]=()=>b.classList.toggle('y');d[o]=()=>c[v]=''};</script><style>.y{text-decoration:line-through}

Combining yours & the strike thru example, managed to get both strike thru & clear added in 277 chars. If i could save another char could close the p which would be nice :p

https://jsfiddle.net/hysjgkcm/

Down to 270 https://jsfiddle.net/vr7246jk/1/