README: updated todo

README.md

@@ -376,6 +376,14 @@ So presumably, according to caniuse.com,
 
 ## Remaining tasks
 
+* Use HTML `<template>`s for rendering state messages,
+    instead of having them all in HTML and then hiding irrelevant with CSS,
+    for better graceful degradation in browsers that don't support complex CSS queries with `:not`.
+* Implement custom progressive form wrapper element (use `MutationObserver` to find when forms are added/removed),
+    use it instead of custom built-in element in browsers not supporting custom built-in elements
+    (i.e. all modern WebKit browsers including Safari 10.1+, and Chrome 54 to 66),
+    so that these browsers will get offline client-only support too,
+    instead of falling back to client-server mode.
 * Implement error handling and handling of incorrect / malformed game states (since they come from the client).
 * Improve UI for large boards (disable autoplayers when board is too large,
     hide "enable autoplayer" buttons, provide clear indication to the user instead).