We can achieve this using one gradient like below:
.plus {
--b: 4px; /* the thickness */
width: 40px; /* the size */
aspect-ratio: 1;
border: 10px solid #000; /* the outer space */
background:
conic-gradient(from 90deg at var(--b) var(--b),#000 90deg,#fff 0)
calc(100% + var(--b)/2) calc(100% + var(--b)/2)/
calc(50% + var(--b)) calc(50% + var(--b));
display: inline-block;
}
.alt {
border: none;
margin: 10px;
background:
conic-gradient(from 90deg at var(--b) var(--b),#fff 90deg,#000 0)
calc(100% + var(--b)/2) calc(100% + var(--b)/2)/
calc(50% + var(--b)) calc(50% + var(--b));
}
.radius {
border-radius: 50%;
}<div class="plus">
</div>
<div class="plus alt">
</div>
<div class="plus radius">
</div>Old answer
Use multiple background like below:
.plus {
display:inline-block;
width:50px;
height:50px;
background:
linear-gradient(#fff 0 0),
linear-gradient(#fff 0 0),
#000;
background-position:center;
background-size: 50% 2px,2px 50%; /*thickness = 2px, length = 50% (25px)*/
background-repeat:no-repeat;
}
.alt {
background:
linear-gradient(#000 0 0),
linear-gradient(#000 0 0);
background-position:center;
background-size: 50% 2px,2px 50%; /*thickness = 2px, length = 50% (25px)*/
background-repeat:no-repeat;
}
.radius {
border-radius:50%;
}<div class="plus">
</div>
<div class="plus alt">
</div>
<div class="plus radius">
</div>
And here is with transparency:
.plus {
width:50px;
height:50px;
display:inline-block;
background:
linear-gradient(#000 0 0) top left,
linear-gradient(#000 0 0) top right,
linear-gradient(#000 0 0) bottom left,
linear-gradient(#000 0 0) bottom right;
background-size: calc(50% - 1px) calc(50% - 1px); /*thickness = 2px (2*1px) */
background-repeat:no-repeat;
border:10px solid #000; /*length = 30px (50px - 2x10px) */
box-sizing:border-box;
}
.radius {
border-radius:50%;
}
body {
background:pink;
}<div class="plus">
</div>
<div class="plus radius">
</div>
We can add CSS variable to easily control the overal shape:
.plus {
--t:2px; /* Thickness */
--l:40px; /* size of the symbol */
--s:10px; /* space around the symbol */
--c1:#fff; /* Plus color*/
--c2:#000; /* background color*/
display:inline-block;
width:var(--l);
height:var(--l);
padding:var(--s);
box-sizing:border-box; /*Remove this if you don't want space to be included in the size*/
background:
linear-gradient(var(--c1) 0 0) content-box,
linear-gradient(var(--c1) 0 0) content-box,
var(--c2);
background-position:center;
background-size: 100% var(--t),var(--t) 100%;
background-repeat:no-repeat;
}
.radius {
border-radius:50%;
}<div class="plus"></div>
<div class="plus" style="--l:35px;--t:3px;--c2:green"></div>
<div class="plus" style="--l:50px;--t:1px;--s:5px;--c1:red;"></div>
<div class="plus" style="--l:35px;--t:5px;--s:0px;--c1:blue;--c2:orange;"></div>
<br>
<div class="plus radius"></div>
<div class="plus radius" style="--l:35px;--t:3px;--c2:green"></div>
<div class="plus radius" style="--l:50px;--t:1px;--s:5px;--c1:red;"></div>
<div class="plus radius" style="--l:35px;--t:5px;--s:0px;--c1:blue;--c2:orange;"></div>