diff --git src/wp-includes/kses.php src/wp-includes/kses.php
index 2b96e84a47..198b0fd7cb 100644
--- src/wp-includes/kses.php
+++ src/wp-includes/kses.php
@@ -2063,6 +2063,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 	 * @since 5.0.0 Added support for `background-image`.
 	 * @since 5.1.0 Added support for `text-transform`.
 	 * @since 5.2.0 Added support for `background-position` and `grid-template-columns`
+	 * @since 5.3.0 Added support for `flex-basis`.
 	 *
 	 * @param string[] $attr Array of allowed CSS attributes.
 	 */
@@ -2141,6 +2142,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 			'vertical-align',
 			'list-style-type',
 			'grid-template-columns',
+			'flex-basis',
 		)
 	);