Add flag for experimental header filtering

Author: JayPanoz

css/ReadiumCSS-config.css

@@ -50,4 +50,5 @@
 @custom-selector :--ipadOS-patch [style*="readium-iPadOSPatch-on"];
 
 /* Experimental/Feature flags */
-@custom-selector :--experimental-zoom [style*="readium-experimentalZoom-on"];
+@custom-selector :--experimental-zoom [style*="readium-experimentalZoom-on"];
+@custom-selector :--experimental-header-filtering [style*="readium-experimentalHeaderFiltering-on"]

css/src/modules/user-settings-submodules/ReadiumCSS-paraIndent_pref.css

@@ -6,7 +6,8 @@
 
    Repo: https://github.com/readium/css */
 
-:root[style*="--USER__paraIndent"] p:not([class*="title"]):not(blockquote p):not(figcaption p):not(header p):not(hgroup p):not(div:has(+ *) > h1 + p):not(div:has(+ *) > p:has(+ h1)) {
+:root[style*="--USER__paraIndent"] p:not(blockquote p):not(figcaption p):not(header p):not(hgroup p),
+:root[style*="--USER__paraIndent"]:--experimental-header-filtering p:not([class*="title"]):not(div:has(+ *) > h1 + p):not(div:has(+ *) > p:has(+ h1)) {
   text-indent: var(--USER__paraIndent) !important;
 }
 

css/src/modules/user-settings-submodules/ReadiumCSS-textAlign_pref.css

@@ -11,9 +11,10 @@
 }
 
 :root[style*="--USER__textAlign"] body,
-:root[style*="--USER__textAlign"] p:not([class*="title"]):not(blockquote p):not(figcaption p):not(header p):not(hgroup p):not(div:has(+ *) > h1 + p):not(div:has(+ *) > p:has(+ h1)),
+:root[style*="--USER__textAlign"] p:not(blockquote p):not(figcaption p):not(header p):not(hgroup p),
 :root[style*="--USER__textAlign"] li,
-:root[style*="--USER__textAlign"] dd {
+:root[style*="--USER__textAlign"] dd,
+:root[style*="--USER__textAlign"]:--experimental-header-filtering p:not([class*="title"]):not(div:has(+ *) > h1 + p):not(div:has(+ *) > p:has(+ h1)) {
   text-align: var(--USER__textAlign) !important;
   -moz-text-align-last: auto !important;
   -epub-text-align-last: auto !important;