You can use SVGs to connect two divs using only HTML and CSS:
<div id="div1" style="width: 100px; height: 100px; top:0; left:0; background:#777; position:absolute;"></div>
<div id="div2" style="width: 100px; height: 100px; top:300px; left:300px; background:#333; position:absolute;"></div>
(please use seperate css file for styling)
Create a svg line and use this line to connect above divs
<svg width="500" height="500"><line x1="50" y1="50" x2="350" y2="350" stroke="black"/></svg>
where,
x1,y1 indicates center of first div and
x2,y2 indicates center of second div
You can check how it looks in the snippet below
<div id="div1" style="width: 100px; height: 100px; top:0; left:0; background:#777; position:absolute;"></div>
<div id="div2" style="width: 100px; height: 100px; top:300px; left:300px; background:#333; position:absolute;"></div>
<svg width="500" height="500"><line x1="50" y1="50" x2="350" y2="350" stroke="black"/></svg>
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