[KevinH]

I added some debug that captures the css input to the denester and the css outout after converting back to a QString and here are the differences:

diff -u input.txt output.txt
--- input.txt	2026-06-09 13:07:12
+++ output.txt	2026-06-09 13:06:13
@@ -14,24 +14,31 @@
     page-break-before: always;
     text-align: justify;
     }
+
 @font-face {
+
     font-family: "Helvetica Neue";
     font-style: normal;
     font-weight: normal;
     src: url(../Fonts/HelveticaNeue-Medium.otf);
     }
+
 @font-face {
+
     font-family: "Helvetica Neue";
     font-style: normal;
     font-weight: bold;
     src: url(../Fonts/HelveticaNeueBold.ttf);
     }
+
 @font-face {
+
     font-family: "Franklin Gothic Std Extra Cond";
     font-style: normal;
     font-weight: normal;
     src: url(../Fonts/FranklinGothicStd-ExtraCond.otf);
     }
+
 p {
     display: block;
     margin-top: 0;
@@ -40,6 +47,7 @@
     margin-right: 0;
     line-height: 1.15em;
     }
+
 .calibre1 {
     display: block;
     margin-bottom: 0;
@@ -49,11 +57,13 @@
     text-indent: 1.5em;
     text-align: justify;
     }
+
 blockquote {
     margin: 1em 2em;
     padding: 0;
     display: block;
     }
+
 blockquote.bqfc {
     margin: 1em auto;
     padding: 0 2em;
@@ -61,39 +71,49 @@
     width: fit-content;
     box-sizing: border-box;
     }
+
 blockquote.bqc {
     margin: 1em 0;
     padding: 0;
     display: block;
     }
+
 .bold {
     font-weight: bold;
     }
+
 .center {
     text-align: center;
     }
+
 .right {
     text-align: right;
     }
+
 .left {
     text-align: left;
     }
+
 .image {
     margin-top: 0.5em;
     margin-bottom: 0.5em;
     text-align: center;
     }
+
 .image1 {
     height: 1em;
     }
+
 .justify {
     text-align: justify;
     }
+
 .noheading {
     margin-top: 6em;
     margin-bottom: 0;
     text-align: justify;
     }
+
 .none {
     margin-top: 0;
     margin-bottom: 0;
@@ -101,14 +121,17 @@
     margin-right: 0;
     text-align: justify;
     }
+
 .separator {
     margin-bottom: 0.5em;
     margin-top: 0.5em;
     text-align: center;
     }
+
 .smc {
     font-variant: small-caps;
     }
+
 .heading1 {
     font-size: 2em;
     font-style: italic;
@@ -117,6 +140,7 @@
     margin-bottom: 1.2em;
     text-align: center;
     }
+
 .heading2 {
     font-size: 1.45em;
     font-style: italic;
@@ -124,6 +148,7 @@
     margin-bottom: 1em;
     text-align: center;
     }
+
 .hindent {
     padding-left: 1em;
     text-indent: -1em;
@@ -133,12 +158,14 @@
     margin-right: 0;
     text-align: left;
     }
+
 hr {
     border: 0;
     background-color: #000000;
     height: 4px;
     width: 85%;
     }
+
 hr.hrsep {
     border: 0;
     background-color: #000000;
@@ -147,26 +174,31 @@
     margin-top: 0.75em;
     margin-bottom: 0.75em;
     }
+
 br.brheight {
     content: " ";
     display: block;
     margin: 0.5em 0;
     }
+
 p.ra {
     float: right;
     margin-bottom: 0;
     margin-top: 0;
     text-align: right;
     }
+
 p.la {
     float: left;
     margin-bottom: 0;
     margin-top: 0;
     text-align: left;
     }
+
 div.noheight {
     line-height: 0;
     }
+
 .border {
     border-style: solid;
     border-width: 3px;
@@ -175,18 +207,21 @@
     padding-top: 2px;
     padding-bottom: 2px;
     }
+
 td.tlhs {
     margin: 0;
     padding: 0;
     text-align: left;
     vertical-align: top;
     }
+
 td.trhs {
     padding: 0 0 0 0.5em;
     margin: 0;
     text-align: left;
     vertical-align: top;
     }
+
 span.drop {
     float: left;
     font-size: 3.1em;
@@ -196,6 +231,7 @@
     margin-bottom: -0.25em;
     margin-right: 0.03em;
     }
+
 span.drop1 {
     float: left;
     font-size: 100%;
@@ -205,9 +241,11 @@
     margin-bottom: -0.25em;
     margin-right: 0.15em;
     }
+
 span.font {
     font-size: 0.8em;
     }
+
 .copyright {
     font-size: 0.9em;
     margin-top: 35%;
@@ -216,30 +254,36 @@
     padding-right: 5%;
     text-align: left;
     }
+
 .copyright1 {
     margin-top: 1.2em;
     margin-bottom: 0;
     }
+
 .cimg {
     height: 1.75em;
     vertical-align: -30%;
     }
+
 .dedic {
     margin-top: 18%;
     text-align: center;
     }
+
 .title {
     text-indent: 0;
     text-align: center;
     width: auto;
     margin: 2.5% 0 0;
     }
+
 .titleimg {
     max-width: 100%;
     max-height: 13em;
     margin: 0 auto;
     display: block;
     }
+
 .title1 {
     font-family: "Franklin Gothic Std Extra Cond";
     font-size: 3em;
@@ -247,6 +291,7 @@
     line-height: 1em;
     text-align: center;
     }
+
 .title2 {
     font-family: "Helvetica Neue";
     font-weight: bold;
@@ -255,10 +300,12 @@
     line-height: 1em;
     text-align: center;
     }
+
 .title3 {
     margin-top: 0.1em;
     text-align: center;
     }
+
 .title4 {
     font-family: "Helvetica Neue";
     font-size: 0.65em;
@@ -266,6 +313,7 @@
     line-height: 1em;
     text-align: center;
     }
+
 .title5 {
     font-family: "Helvetica Neue";
     font-size: 0.8em;
@@ -273,14 +321,20 @@
     line-height: 1em;
     text-align: center;
     }
+
 .titleimg3 {
     height: 2.75em;
     }
+
 .arthead {
     margin-bottom: 0.75em;
     text-align: center;
     }
+
 .vale {
     margin-top: 1em;
     text-align: left;
     }
+
+
+

Notice how newlines are being added in different places, some after the selector's opening { and others after the selectors closing }.

I am not sure how this change is happening, and what characters are
causing this but it at least shows me that something funny is going on.