Le fichier Json était mal formé.
Voici la version correcte :
{
"style_formats": [{
"title": "Bold text",
"inline": "b"
},
{
"title": "Red text",
"inline": "span",
"styles": {
"color": "#ff0000"
}
},
{
"title": "Red header",
"block": "h1",
"styles": {
"color": "#ff0000"
}
}
],
"paste_retain_style_properties": "margin, padding, width, height, font-size,font-weight,font-family,color,text-align,ul,ol,li,text-decoration,border,background,float,display"
}
A+ ;)