@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;