Тонкая рамка border вокруг блока тоньше 1px

div { 
  width: 100px; 
  height: 100px; 
  box-shadow: 0 0 1px 0  
}

<div></div>

body { 
  display: flex; 
  flex-wrap: wrap; 
} 
div { 
  width: 200px; 
  height: 200px; 
  margin: 10px; 
  justify-content: center; 
  align-items: center; 
  display: flex; 
  text-align: center; 
} 
.b-1 { 
  border: 1px solid black; 
} 
 
.b-2 { 
  border: 1px solid darkgrey; 
} 
 
.b-3 { 
  border: 1px solid grey; 
} 
 
.b-4 { 
  border: 1px solid lightgrey; 
} 
 
.b-5 { 
  box-shadow: 0 0 1px 0; 
} 
 
.b-6 { 
  box-shadow: 0 0 1px 0 rgba(0,0,0,0.25); 
} 
 
.b-7 { 
  box-shadow: 0 0 1px 0 lightgray; 
}

<div class="b-1"><span>border: 1px solid black;</span></div> 
<div class="b-2"><span>border: 1px solid darkgrey;</span></div> 
<div class="b-3"><span>border: 1px solid grey;</span></div> 
<div class="b-4"><span>border: 1px solid lightgrey;</span></div> 
<div class="b-5"><span>box-shadow: 0 0 1px 0;</span></div> 
<div class="b-6"><span>box-shadow: 0 0 1px 0 rgba(0,0,0,0.25);</span></div> 
<div class="b-7"><span>box-shadow: 0 0 1px 0 lightgray;</span></div>