Index: shortcodes/class-readme-validator.php
===================================================================
--- shortcodes/class-readme-validator.php	(revision 14619)
+++ shortcodes/class-readme-validator.php	(working copy)
@@ -37,6 +37,22 @@
 
 		ob_start();
 		?>
+        <style>
+            .readme-validator-input,
+            .readme-validator-textarea {
+                width: 100%;
+                max-width: 100%;
+                box-sizing: border-box;
+            }
+
+            .readme-validator-actions .readme-validator-input {
+                display: block;
+            }
+
+            .readme-validator-actions .wp-element-button {
+                margin-top: 8px;
+            }
+        </style>
 		<div class="wrap">
 			<?php
 			if ( $readme_contents || $readme_url ) {
@@ -46,15 +62,26 @@
 			}
 			?>
 
-			<form method="get" action="">
-				<p>
-					<input type="text" name="readme" size="70" placeholder="https://" value="<?php echo esc_attr( $readme_url ); ?>" />
-					<input type="submit" class="wp-element-button button" value="<?php esc_attr_e( 'Validate!', 'wporg-plugins' ); ?>" />
-				</p>
-			</form>
+            <form method="get" action="">
+                <p class="readme-validator-actions">
+                    <input
+                            type="text"
+                            name="readme"
+                            size="70"
+                            placeholder="https://"
+                            value="<?php echo esc_attr( $readme_url ); ?>"
+                            class="readme-validator-input"
+                    />
+                    <input
+                            type="submit"
+                            class="wp-element-button button"
+                            value="<?php esc_attr_e( 'Validate!', 'wporg-plugins' ); ?>"
+                    />
+                </p>
+            </form>
 
 			<p><?php _e( '... or paste your <code>readme.txt</code> here:', 'wporg-plugins' ); ?></p>
-				<textarea rows="20" cols="100" name="readme_visible" placeholder="=== Plugin Name ==="><?php echo esc_textarea( $readme_contents ); ?></textarea>
+				<textarea rows="20" cols="100" name="readme_visible" placeholder="=== Plugin Name ===" class="readme-validator-textarea"><?php echo esc_textarea( $readme_contents ); ?></textarea>
 				<form id="readme-data" method="post" action="">
 					<input type="hidden" name="readme" value="" />
 					<textarea class="screen-reader-text" rows="20" cols="100" name="readme_contents"><?php echo esc_textarea( $readme_contents ); ?></textarea>