Look here what I did. It might be interesting for other Cesium newbies, too.
1--I smoothed my GPX-file (from Logger GP102) in
http://www.gpsvisualizer.com/convert_input?convert_format=gpx
2--I noticed that my CZML was underground or hidden in the wood.
So I had to increase the elevations in my GPX (20 meters,30 meters, and at least 75 meters added.
I did this with help of
3--The "lifted" GPX I made to a CZML file as shown in
https://willgeary.github.io/PyConGhana/
In the generated CZML i changed
"heightReference": "RELATIVE_TO_GROUND"
into
"heightReference": "CLAMP_TO_GROUND"
This change applies to the "leading blue circle" in the map.
4--Then I copied the CZML into
https://glitch.com/edit/#!/2016-09-15-obere-firstalm (shared)
as proposed by O.Shehata --->var czml = .....
5--Now look at
https://2016-09-15-obere-firstalm.glitch.me/
and move the "time pointer" at the bottom.
Please consider that I have poor knowledge in scripting (HTML,PHP,JS,Python). I am much better in copy/paste and in asking other people.
I found Cesium in
https://help.openstreetmap.org/questions/66711/this-dynamic-3d-map-uses-openstreetmap-too-what-tools-can-create-such-a-map
Josef