diff --git a/src/MDMailFormat.php b/src/MDMailFormat.php
index 2d3bf5a..a746b21 100644
--- a/src/MDMailFormat.php
+++ b/src/MDMailFormat.php
@@ -10,29 +10,29 @@ declare(strict_types = 1);
  */
 final class MDMailFormat {
 
-    const ACCENT_COLOR_MAIN = "#0280ee";
-    const ACCENT_COLOR_SEC  = "#0d5dc8";
-    const CSS_H1 = "margin: .5em 0 .8em 0; padding: 0 0 .5em 0; font-size: 1.3em; font-weight: bold; border-bottom: 1px solid #D6D6D6;";
-    const CSS_H2 = "margin: .3em 0 .5em 0; padding: 0 0 0 0; font-size: 1.1em; font-weight: bold;";
-    const CSS_P = "margin: 0 0 0 0; padding: .4em 0 .6em 0; font-style: normal;";
-    const CSS_FOOTER = "margin-top: .5em; padding-top: .5em; border-top: .1em solid #D6D6D6;";
-    const CSS_UL = "margin: 1em 0; padding: 0 0 0 1em; list-style: square;";
-    const CSS_OL = "margin: 1em 0; padding: 0 0 0 1em;";
-    const CSS_HR = "margin: 1em 0; border-width: 0 0 0 0; border-top: 1px solid #D6D6D6;";
-    const CSS_A_NORMAL = "text-decoration: none;";
-    const CSS_A_FOOTER = "text-decoration: none; font-size: .95em;";
-    const CSS_A_FOOTER_HOVER = "color: " . self::ACCENT_COLOR_SEC . " !important;";
-    const CSS_A_BUTTONLIKE = 'display: inline-block; width: auto;
+    private const ACCENT_COLOR_MAIN = "#0280ee";
+    private const ACCENT_COLOR_SEC  = "#0d5dc8";
+    private const CSS_H1 = "margin: .5em 0 .8em 0; padding: 0 0 .5em 0; font-size: 1.3em; font-weight: bold; border-bottom: 1px solid #D6D6D6;";
+    private const CSS_H2 = "margin: .3em 0 .5em 0; padding: 0 0 0 0; font-size: 1.1em; font-weight: bold;";
+    private const CSS_P = "margin: 0 0 0 0; padding: .4em 0 .6em 0; font-style: normal;";
+    private const CSS_FOOTER = "margin-top: .5em; padding-top: .5em; border-top: .1em solid #D6D6D6;";
+    private const CSS_UL = "margin: 1em 0; padding: 0 0 0 1em; list-style: square;";
+    private const CSS_OL = "margin: 1em 0; padding: 0 0 0 1em;";
+    private const CSS_HR = "margin: 1em 0; border-width: 0 0 0 0; border-top: 1px solid #D6D6D6;";
+    private const CSS_A_NORMAL = "text-decoration: none;";
+    private const CSS_A_FOOTER = "text-decoration: none; font-size: .95em;";
+    private const CSS_A_FOOTER_HOVER = "color: " . self::ACCENT_COLOR_SEC . " !important;";
+    private const CSS_A_BUTTONLIKE = 'display: inline-block; width: auto;
         margin: .5em 0; padding: .6em .8em;
         text-decoration: none;
         font-size: inherit; font-weight: bold;
         border-radius: .2em; transition: border .2s;
         background: ' . self::ACCENT_COLOR_MAIN . ' !important; color: #FFF !important;
         transition: background .4s;';
-    const CSS_A_BUTTONLIKE_HOVER = 'background: ' . self::ACCENT_COLOR_SEC . ' !important;';
+    private const CSS_A_BUTTONLIKE_HOVER = 'background: ' . self::ACCENT_COLOR_SEC . ' !important;';
 
-    const CSS_LOGO = "height: 1.5em; margin-right: .4em; vertical-align: middle;";
-    const CSS_H1_SPAN = "vertical-align: middle;";
+    private const CSS_LOGO = "height: 1.5em; margin-right: .4em; vertical-align: middle;";
+    private const CSS_H1_SPAN = "vertical-align: middle;";
 
     /** @var string */
     private string $_msg_html = "";