Все свойства css в json

document.querySelectorAll('.block').forEach(el => el.onclick = e => { 
  let dummy = document.createElement(el.tagName); 
  document.body.append(dummy); 
  let dummyStyles = getComputedStyle(dummy); 
  let styles = Object.entries(getComputedStyle(el)); 
  ta.textContent = styles.filter(s => s[1] !== dummyStyles[s[0]]) 
                         .map(s => s.join(': ')) 
                         .join('\n') 
  dummy.remove();                        
})

.block{ 
  width:50px; 
  height:50px; 
  box-sizing: border-box; 
  display: inline-block; 
} 
 
.one { 
  background-color:red; 
} 
 
.two { 
  border:solid; 
} 
 
#ta { 
  height:115px; 
  width:550px; 
}

<textarea id=ta ></textarea><br> 
<div tabindex="1" class="block one"></div> 
<div tabindex="2" class="block two"></div>