Error if using CDN
When you insert your icons via css/cdn (https://cdn.jsdelivr.net/npm/boxicons@latest/css/boxicons.min.css), chrome responds with an ERR_INTERNET_DISCONNECTED error message (as if you were offline). Digging (not so deep) into this, turns out that it refers to Issue [1173575: Remove remaining usages of cr.js and other non-module shared files](url), so you have to go either with a local copy or via web components.