@sam i did it with an nginx rule!

map $http_user_agent $brnotice {
    default         '</head>';
    ~*Chrom         '<link rel="stylesheet" href="https://maple.pet/notice.css"></head><div class="impnotice">You are using an unsupported browser. Please visit <a href="https://getfirefox.com">this website</a> if you would like to upgrade to a supported browser.</div>';
}

and in the server block:

        sub_filter '</head>' $brnotice;
        sub_filter_once on;