This happens only in https://playground.wordpress.net/, when run locally as VS code extension, the issue does not appear.

Block inserter "+" icon is not styled; some other UI elements (like buttons when inserting new image block, buttons on starter template on columns block, etc) are not styled as well; when inserting new image and uploading it through interface, the image does not show in block editor.

Tested on Firefox developer version 118 b9 and Chrome dev version 118 on Windows 10.

Steps to reproduce:
Start new WP instance in playground. Create new post, insert image from local file.

Related

• https://bugs.chromium.org/p/chromium/issues/detail?id=880768
• https://bugzilla.mozilla.org/show_bug.cgi?id=1293277
• serviceworker for iframes with srcdoc w3c/ServiceWorker#765
• CSS files are not loading in the site editor #42
• b7ca737