subframe-that-posts-html-containing-blob-url-to-parent.html

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<!DOCTYPE html>

<meta charset="utf-8">

<script>

fetch('html-correctly-labeled.html')

    .then(response => response.blob())

    .then(blob => {

        let msg = { blob_size: blob.size,

                    blob_type: blob.type,

                    blob_url: URL.createObjectURL(blob) };

        window.parent.postMessage(msg, '*');

})

    .catch(error => {

        let msg = { error: error };

        window.parent.postMessage(msg, '*');

});

</script>