[stumped]

can someone explain this error message
I have a epub which give this message for the validate stylesheets... tool

the well formed... f7 check says all OK

the book styles look ok by eye. But if i do my usual clean up routine of an epub to epub conversion in calibre, then sigil will not open the output file from that. I can see it running in talk manager, but i never get the output window. Sigil will happily open the original file

here is the offending stylesheet:

Spoiler:

h1 {
    font-weight : normal;
    font-style : normal;
}

h2 {
    font-weight : normal;
    font-style : normal;
}

h3 {
    font-weight : normal;
    font-style : normal;
}

h4 {
    font-weight : normal;
    font-style : normal;
}

h5 {
    font-weight : normal;
    font-style : normal;
}

div {
    margin-top : 0em;
    margin-bottom : 0em;
    margin-left : 0em;
    margin-right : 0em;
}

p {
    text-indent : 0em;
}

a {
    text-decoration : none;
    color : inherit;
}

.spacebreak {
    margin : 0;
}

div.informalfigure {
    page-break-inside : avoid;
}

table {
    border-collapse : collapse;
}

h4.titlechapterprologue {
margin-bottom : 1.4em;
        text-align : center;
        font-weight : bold;
        }
p.chapterprologuefirstpara {
text-align : justify;
        }

p.chapterprologue {
text-align : justify;
        text-indent : 1.5em;
        }
p.chapterprologuenoindent {
text-align : justify;
        }
h4.chaptermark {
margin-bottom : 1.4em;
        text-align : center;
        font-weight : bold;
        }
p.chapterfirstpara {
text-align : justify;
        }

p.chapter {
text-align : justify;
        text-indent : 1.5em;
        }
p.chapternoindent {
text-align : justify;
        }

h4.titlechapterepilogue {
margin-bottom : 1.4em;
        text-align : center;
        font-weight : bold;
        }
p.chapterepiloguefirstpara {
text-align : justify;
        }

p.chapterepilogue {
text-align : justify;
        text-indent : 1.5em;
        }