How to add a dotted line or add padding to td border

The below image is already close to the solution i am looking for (see my codepen). The only thing that is missing is a padding / gap between the outline border of <tbody> or <thead> and the border of the <td>

Questions

What approach would you take if you had to create something that looks like the image above?
How can i achieve that the dotted lines have some gap between the border of the td (/ tr) and the outline of either tr or tbody or table?

Code

You can take a look at the code on codepen. My CSS approach is this

table {  width: 100%; }
/** outline: 0.5pt solid; outline-color: black; border-spacing: .2em 0em; **/

thead, tbody { outline: 0.5pt solid; outline-color: black; }

th.sgn { border-bottom: 1pt dotted blue; padding-top: 1ex;}    
td.sgn { border-bottom: 1pt dotted blue; padding-top: 1ex;}

<table>
       <colgroup>
         <col width="20%">
         <col width="35%">                    
         <col width="15%">
         <col width="30%">                    
    </colgroup>
    <thead>
       <tr>
            <th>Werkzeugnr:</th>
            <th class="sgn"> &nbsp; </th>
            <th>Index</td><th class="sgn"></th>
       </tr>                            
       <tr class="sml">
            <th colspan="4"> &nbsp; </th>
       </tr>                            
    </thead>
    <tbody>
        <tr><th colspan="4"><h3>Bitte ausfüllen und abheften!</h3></th></tr>
    </tbody>
    <tbody>                                                
       <tr>
         <td>Einbaudatum:</td><td class="sgn"> &nbsp; </td>
         <td>Name</td><td class="sgn"> &nbsp;</td>
       </tr>
       <tr><td>Anlaufprobleme:</td><td><input type="checkbox"></td><td colspan="2">Bitte Grund angeben</td>
       </tr>
    </tbody>    
</table>

Solution

I would achieve that spacing with ::before and ::after pseudo classes.

First, give elements you want to give space a unique class (e.g. .spaced).

<td class="spaced sgn" colspan="3"> &nbsp; </td>

Then, give it a relative position so ::before and ::after get positioned relative to it:

.spaced {
  position: relative;
}

Add ::before and ::after with the width you want as space, and position them to left and right respectively:

.spaced::before,
.spaced::after {
  content: "";
  position: absolute;
  width: 1rem;
  height: 1rem;
  background: white;
  bottom: -0.5rem;
}

.spaced::before {
  left: 0;
}

.spaced::after {
  right: 0;
}

CodePen: https://codepen.io/moaaz_bs/pen/xxPJoQL?editors=1100