still awaiting a webquiz (or tikz) update that includes the new pgfsys-dvisvgm4ht.def to update in TL ...

I think this looks like a bug. Reporting just in case.

reference

https://tex.stackexchange.com/questions/488880/error-from-pgf-libraries-pgflibraryfadings-code-tex-when-adding-tcolorbox-tl

Just installed fresh TL 2019. I was trying the new tikz driver for tex4ht. I found this error. Here is a MWE. This error only shows up in tex4ht and not in lualatex and it shows up only when adding tcolorbox package

I actually do not use \usepackage[most]{tcolorbox} much any more. This was in an old file. So I can remove this package for now. But thought to mention this error just in case it affects other code somewhere else.

Here is MWE

-------------------
\documentclass[11pt]{article}

\ifdefined\HCode %This driver to tex4ht only. For tikz
\def\pgfsysdriver{pgfsys-dvisvgm4ht.def}
\fi
\usepackage{tikz,graphicx,tikz-dependency}
\usetikzlibrary{fit}
\usepackage{adjustbox}
\usepackage[most]{tcolorbox} %adding this causes the error
%\tcbuselibrary{listings}
\begin{document}

\adjustbox{max width=1.35in}{
\begin{tikzpicture}[baseline=(current bounding box.center)]
\coordinate[label =above:$0$, label =below:{$u=0$}] (A) at (0,0);
\coordinate[label =above:$\pi$, label =below:{$u_x=0$}] (B) at (4,0);

\draw (A) -- node[above] {$u_t = k u_{xx} +c^2 u_x $} node[below] {$f(x)$} ++ (B);
\end{tikzpicture}
}
\end{document}
------------------------------

Gives error

-------------------
(/usr/local/texlive/2019/texmf-dist/tex/generic/pgf/libraries/pgflibraryfadings
.code.tex
! Undefined control sequence.
\a:tikzpicture ->\Picture
*{}
l.31 ...ing{east}{\pgfuseshading{pgf@lib@fade@east}}
%
?
--------------------------

TL 2019 on Linux

which make4ht
/usr/local/texlive/2019/bin/x86_64-linux/make4ht
which lualatex
/usr/local/texlive/2019/bin/x86_64-linux/lualatex

Thanks
--Nasser