diff --git a/01.courses/01.optics/03.the-nature-of-light/03.annex/annex.fr.md b/01.courses/01.optics/03.the-nature-of-light/03.annex/annex.fr.md
index a8203417c..18b6e75ff 100644
--- a/01.courses/01.optics/03.the-nature-of-light/03.annex/annex.fr.md
+++ b/01.courses/01.optics/03.the-nature-of-light/03.annex/annex.fr.md
@@ -10,15 +10,6 @@ slug: annexe
 La [Constante $c$](#une-constante-fondamentale-de-la-nature-la-vitesse-de-la-lumière).
 
 
-[Current chapter](.)
-
-[Parent chapter](..)
-
-[Sibling chapter](../cheatsheet)
-
-[Child chapter](cheatsheet)
-
-
 ### Unrelated image
 
 ![Showing rays for different focal points](rays-when-focal-point-moves.gif)
diff --git a/test/default.md b/test/default.md
index 77b421703..ef2cffcb5 100644
--- a/test/default.md
+++ b/test/default.md
@@ -4,10 +4,12 @@
 0. `*`: *star*
 1. `**`: **globstar**
 2. `_`: _underscore_
-3. `__`: __init__
-5. `~~`: ~~double tilde~~
+3. `__`: __underscores__
+4. `~~`: ~~double tilde~~
 5. `~`: tilde
 
+#### Definition list
+
 <!-- definition lists -->
 celerity $c$
 : speed of light in a vacuum
@@ -39,4 +41,21 @@ e^{i\tau}=1
 ```
 
 > The complex exponential of the circle constant is unity.
->> $e^{i\tau}=1$
\ No newline at end of file
+>> $e^{i\tau}=1$
+
+### GeoGebra
+
+#### Iframe
+
+<!-- https://wiki.geogebra.org/en/Reference:Material_Embedding_(Iframe) -->
+<iframe class="geogebra" src="https://www.geogebra.org/material/iframe/id/wkrw5qgm/sfsb/true" fullscreen allowfullscreen width="400px" height="280px"></iframe>
+
+#### ???
+
+### Videos
+
+#### Iframe
+
+<iframe width="560" height="315" sandbox="allow-same-origin allow-scripts" src="https://video.samedi.pm/videos/embed/c06dbd9e-d8c7-4655-aade-51ae95b998c3" frameborder="0" allowfullscreen></iframe>
+
+