You can simulate a readonly select box using the CSS pointer-events property:
select[readonly]
{
pointer-events: none;
}
The HTML tabindex property will also prevent it from being selected by keyboard tabbing:
<select tabindex="-1">
select[readonly]_x000D_
{_x000D_
pointer-events: none;_x000D_
}_x000D_
_x000D_
_x000D_
/* irrelevent styling */_x000D_
_x000D_
*_x000D_
{_x000D_
box-sizing: border-box;_x000D_
}_x000D_
_x000D_
*[readonly]_x000D_
{_x000D_
background: #fafafa;_x000D_
border: 1px solid #ccc;_x000D_
color: #555;_x000D_
}_x000D_
_x000D_
input, select_x000D_
{_x000D_
display:block;_x000D_
width: 20rem;_x000D_
padding: 0.5rem;_x000D_
margin-bottom: 1rem;_x000D_
}
_x000D_
<form>_x000D_
<input type="text" value="this is a normal text box">_x000D_
<input type="text" readonly value="this is a readonly text box">_x000D_
<select readonly tabindex="-1">_x000D_
<option>This is a readonly select box</option>_x000D_
<option>Option 2</option>_x000D_
</select>_x000D_
<select>_x000D_
<option>This is a normal select box</option>_x000D_
<option>Option 2</option>_x000D_
</select>_x000D_
</form>
_x000D_