use RTL to shift \llap content and fix \llap, \rlap extreme wrapping

lib/LaTeXML/Engine/plain.pool.ltxml

@@ -536,9 +536,10 @@ DefConstructor('\ltx@centerline{}', sub {
   alias   => '\centerline',
   bounded => 1);
 
-# These should be 0 width, but perhaps also shifted?
-DefMacro('\llap{}', '\hbox to 0pt{\hss#1}');
-DefMacro('\rlap{}', '\hbox to 0pt{#1\hss}');
+# rely on RTL trickery to shift the \llap content over the previous text (see CSS)
+# the zero-width space prevents merging the two boxes in a single ltx:text
+DefMacro('\llap{}', '\hbox to 0pt{\@ADDCLASS{ltx_llap}' . "\x{200B}" . '\hbox{\@ADDCLASS{ltx_llap_content}#1}}');
+DefMacro('\rlap{}', '\hbox to 0pt{\@ADDCLASS{ltx_rlap}#1}');
 
 DefMacroI('\m@th', undef, '\mathsurround=0pt ');
 

lib/LaTeXML/resources/CSS/LaTeXML.css

@@ -510,3 +510,15 @@ cite                 { font-style: normal; }
 
 .ltx_overlay {position:relative; }
 .ltx_overlay > span:nth-child(2) {position:absolute; left:0; }
+
+/* \llap, \rlap */
+.ltx_llap, .ltx_rlap {
+  white-space: nowrap; /* width 0pt causes extreme wrapping otherwise */ }
+
+/* shift \llap over the previous text by shifting text direction */
+.ltx_llap {
+  direction: rtl; }
+
+/* restore LTR inside \llap */
+.ltx_llap_content {
+  direction: ltr; }