Use :where

style.css

@@ -22,9 +22,7 @@ header {
 h1 {
   margin: 0
 }
-footer > *,
-header > *,
-main > * {
+:where(footer, header, main) > * {
   margin: 0 auto;
   max-width: 80rem;
   padding: 0 1rem
@@ -62,7 +60,7 @@ main > * {
 }
 
 /* date in calendar */
-.month th, .month td {
+.month :where(th, td) {
   width: 3.5em;
   height: 3.5em;
   border: 1px solid #CBC19A;
@@ -138,9 +136,7 @@ main > * {
   box-shadow: .2em .25em 0 0 RGBA(114, 44, 162,0.5);
   z-index: 1
 }
-.month :focus + label .event-content,
-.month td:hover .event-content,
-.month td:active .event-content {
+.month :where(:focus + label, td:hover, td:active) .event-content {
   visibility: initial
 }