Accessing SVG to extract bounding boxes

I have a local SVG file and want to access the individual bounding boxes of each group, rendered by the browser (and write them to a JSON file).
My html file looks like this:

<!DOCTYPE html>
<html>
    <head>
        <title>SVG Group Access Test</title> 
    </head>
    <body>

        <object data="output.svg" type="image/svg+xml"
         id="alphasvg" width="100%" height="100%"></object>

        <script>
            var a = document.getElementById("alphasvg");

            // It's important to add a load event listener to the object,
            // as it will load the svg doc asynchronously
            a.addEventListener("load", function() {

                // Get the inner DOM of output.svg
                var svgDoc = a.contentDocument;

                // Check if the SVG document is loaded
                if (svgDoc) {
                    // Get all <g> elements within the SVG
                    var groups = svgDoc.querySelectorAll('g');

                    // Iterate over each <g> element
                    groups.forEach((group, index) => {
                        // Log group details to console
                        console.log(`Group ${index + 1}:`, group);

                        // Optionally, get the bounding box of the group
                        var bbox = group.getBBox();
                        console.log(`Bounding box for Group ${index + 1}: x=${bbox.x}, y=${bbox.y}, width=${bbox.width}, height=${bbox.height}`);
                    });
                } else {
                    console.error("Could not access the SVG document.");
                }
            }, false);
        </script>
    </body>
</html>

this html file and output.svg are both in same directory and I open the html with a browser. Despite this, I can not access the SVG (svgDoc is null). I think this has something to do with access policies as described here

I am new to this and want to prevent running a server. I want to call this html file from python as command instead.