Fyi: as I do not know where to store javascript and css file on webflow, I directly imported, in the header, the css code with <style></style> and the js code with <script></script>.
It looks like webflow add the following row in the code under <body>: <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.0/jquery.min.js" type="text/javascript"></script>
This row is in conflict with jquery I use in <head>…
Okay I have found why:
It is because webflow add the jquery in the <body> so I have to move the following from <head> to <body> as well: <script type="text/javascript" src="jquery.simple-text-rotator.js"></script>.