$_SERVER[‘HTTP_USER_AGENT’] is not setup.

When parsing website through https://achecker.ca/checker/index.php (widely used app to check WCAG validity), $_SERVER[‘HTTP_USER_AGENT’] is not set up therefore there is an error on wah-front-functions.php

Quick fix: (hope you add it on next update)

if ( ! function_exists( 'wp_access_helper_body_class' ) ) {
	/**
	 * Add front body classes
	 *
	 * @param  array $classes CSS classes.
	 * @return array          CSS classes
	 */
	function wp_access_helper_body_class( $classes ) {
		global $is_lynx, $is_gecko, $is_IE, $is_opera, $is_NS4, $is_safari, $is_chrome, $is_iphone;
		if ( $is_lynx ) {
			$classes[] = 'lynx';
		} elseif ( $is_gecko ) {
			$classes[] = 'gecko';
		} elseif ( $is_opera ) {
			$classes[] = 'opera';
		} elseif ( $is_NS4 ) {
			$classes[] = 'ns4';
		} elseif ( $is_safari ) {
			$classes[] = 'safari';
		} elseif ( $is_chrome ) {
			$classes[] = 'chrome';
		} elseif ( $is_IE ) {
			$classes[] = 'ie';
			if(isset($_SERVER['HTTP_USER_AGENT'])) {
				if ( preg_match( '/MSIE ( [0-11]+ )( [a-zA-Z0-9.]+ )/', $_SERVER['HTTP_USER_AGENT'], $browser_version ) ) {
					$classes[] = 'ie' . $browser_version[1];
				}
			}
		} else {
			$classes[] = 'unknown';
		}
		if ( $is_iphone ) {
			$classes[] = 'iphone';
		}

		if(isset($_SERVER['HTTP_USER_AGENT']))  {
			if ( stristr( $_SERVER['HTTP_USER_AGENT'], 'mac' ) ) {
				$classes[] = 'osx';
			} elseif ( stristr( $_SERVER['HTTP_USER_AGENT'], 'linux' ) ) {
				$classes[] = 'linux';
			} elseif ( stristr( $_SERVER['HTTP_USER_AGENT'], 'windows' ) ) {
				$classes[] = 'windows';
			}
		}

		$classes[]             = 'wp-accessibility-helper';
		$contrast_setup        = get_option( 'wah_contrast_setup' ) ? get_option( 'wah_contrast_setup' ) : 0;
		$font_setup_type       = get_option( 'wah_font_setup_type' ) ? get_option( 'wah_font_setup_type' ) : 'script';
		$remove_styles_setup   = get_option( 'wah_remove_styles_setup' ) ? get_option( 'wah_remove_styles_setup' ) : 0;
		$location_setup        = get_option( 'wah_left_side' ) ? 'left' : 'right';
		$underline_links_setup = get_option( 'wah_underline_links_setup' ) ? get_option( 'wah_underline_links_setup' ) : 0;
		$wah_left_side         = get_option( 'wah_left_side' );

		if ( $contrast_setup ) {
			$classes[] = 'accessibility-contrast_mode_on'; }
		if ( $font_setup_type ) {
			$classes[] = 'wah_fstype_' . $font_setup_type; }
		if ( $remove_styles_setup ) {
			$classes[] = 'accessibility-remove-styles-setup'; }
		if ( $underline_links_setup ) {
			$classes[] = 'accessibility-underline-setup'; }
		if ( 'left' === $location_setup ) {
			$classes[] = 'accessibility-location-left';
		} else {
			$classes[] = 'accessibility-location-right';
		}
		return $classes;
	}
	add_filter( 'body_class', 'wp_access_helper_body_class' );
}