Finally managed to use my own HTML model so HTML gets rendered correctly. And added alt tags for images to comply wit w3c requirements

lektor/lektordata/project/content/contents.lr

@@ -1,23 +1,25 @@
-_model: HTMLPage
+_model: htmlpage
 ---
 title: Willkommen beim PC Stammertal
 ---
-body:
+html:
 
-<p><br/></p>
-<h3>Unser nächster Anlass: </h3>
-<p><br/></p>
-<p>Sonntag <strong>8. Dezember, Gangfischschiessen</strong> in Ermatingen
-<p><br/></p>
+<br/>
+<br/>
+<h3>Unser nächster Anlass: </h3><br/>
+<br/>
+Sonntag <strong>8. Dezember, Gangfischschiessen</strong> in Ermatingen<br/>
+<br/>
+<br/>
 <div class="threecolumn">
   <div>  
-    <a href="termine/"><img src=" /images/termine_square.jpg"> All unsere Termine</a></p>
+    <a href="termine/"><img src=" /images/termine_square.jpg" alt="Terminkalender"> All unsere Termine</a>
   </div>
   <div>
-    <a href="about/"><img src="/images/about_square.jpg"> Alle Infos über uns</a>
+    <a href="about/"><img src="/images/about_square.jpg" alt="Buch"> Alle Infos über uns</a>
   </div>
   <div>
-    <a href="kontakt/"><img src="/images/kontakt_square.jpg"> Kontaktiere uns</a>
+    <a href="kontakt/"><img src="/images/kontakt_square.jpg" alt="Briefe"> Kontaktiere uns</a>
   </div>
 </div>
 ---

lektor/lektordata/project/models/htmlpage.ini

@@ -1,11 +1,11 @@
 [model]
 name = HTMLPage
-label = {{ this.title }}
+label = htmlpage
 
 [fields.title]
 label = Title
 type = string
 
-[fields.body]
-label = Body
+[fields.html]
+label = HTML
 type = html

lektor/lektordata/project/templates/layout.html

@@ -15,7 +15,7 @@
         <div class="left_header">
           <ul class="nav">
             <li class="{% if this._path == '/' %}active{% endif
-              %}"><a href="{{ '/'|url }}"><img class="header-logo" src="/images/logo.svg">Pistolenclub Stammertal</a></li>
+              %}"><a href="{{ '/'|url }}"><img class="header-logo" src="/images/logo.svg" alt="PCS Logo">Pistolenclub Stammertal</a></li>
           </ul>
         </div>
         <div class="right_header">

lektor/lektordata/project/templates/page.html

@@ -4,5 +4,6 @@
   <div class="inner_page">
     <h2>{{ this.title }}</h2>
     {{ this.body }}
+    {{ this.html }}
   </div>
 {% endblock %}