Joseph Wright suggests to use the Teχ-customary "XX for hex

unicodedomino_kernel_cosmetics.def

@@ -51,7 +51,7 @@
  }%
  \gdef\UTFviii@hexseq#1#2\relax{%
   % display first octet
-  \space 0x\UTFviii@hexbyte{`#1}%
+  \space "\UTFviii@hexbyte{`#1}%
   % recursively handle remaining octets
   \ifx\relax#2\relax\else\UTFviii@hexseq#2\relax\fi%
  }%
@@ -59,7 +59,7 @@
 
 % override message to give the byte in hex
 \def\UTFviii@invalid@err#1{%
- \PackageError{inputenc}{Invalid UTF-8 byte 0x\UTFviii@hexbyte{`#1}}%
+ \PackageError{inputenc}{Invalid UTF-8 byte "\UTFviii@hexbyte{`#1}}%
   \UTFviii@invalid@help%
 }%
 

unicodedomino_kernel_fixup.def

@@ -22,7 +22,7 @@
 %-
 % Fix check for illegal sequences to fail overlong encoded sequences
 % as well as codepoints outside of the Unicode range [0;10FFFF]. Add
-% 0xF4 to the list of permitted lead octets.
+% "F4 to the list of permitted lead octets.
 
 % run the fixup code only if necessary {{{ begin
 \ifx\UTFviii@check@three\@undefined%
@@ -129,7 +129,7 @@
  \unicodedomino@parse@XML@charref%
 }%
 
-% bugfix: add 0xF4 to the list of permitted lead octets
+% bugfix: add "F4 to the list of permitted lead octets
 % (needs the above fix)
 \begingroup%
 \catcode`\~13