Some extra changes

src/components/ButtonLink.astro

@@ -3,12 +3,18 @@ import styles from "../styles/Button.module.css"
 
 export interface Props {
 	href: string
+	title?: string
 	newTab?: boolean
 }
 
-const { href, newTab } = Astro.props
+const { href, newTab, title } = Astro.props
 ---
 
-<a href={href} class={styles.button} target={newTab ? "_blank" : ""}>
+<a
+	href={href}
+	class={styles.button}
+	title={title}
+	target={newTab ? "_blank" : ""}
+>
 	<slot />
 </a>

src/components/Top.astro

@@ -5,12 +5,14 @@ import styles from "../styles/Top.module.css"
 ---
 
 <div class={styles.up} aria-hidden="true" id="up">
-	<ButtonLink href="#"><Icon name="mdi:arrow-up" /></ButtonLink>
+	<ButtonLink href="#" title="Back to top">
+		<Icon name="mdi:arrow-up" />
+	</ButtonLink>
 
 	<script>
 		const up = document.querySelector("#up")
 		document.addEventListener("scroll", () =>
-			up.setAttribute("aria-hidden", window.scrollY == 0)
+			up?.setAttribute("aria-hidden", (window.scrollY == 0).toString())
 		)
 	</script>
 </div>