[tex4ht] what do these warning messages really mean. starts/ends with character code 255

[Nasser M. Abbasi]

When compiling the following MWE, it generates messages such as

warning --- File `lm-ec.htf' starts/ends with character code 255 (instead of 124)
warning --- File `lm-ec.htf' starts/ends with character code 255 (instead of 124)
warning --- Couldn't find font `zxlri-8r.htf' (char codes: 17--181)
--- Couldn't find font `zxlr-8r.htf' (char codes: 17--181)
etc..

But when looking at the HTML output, there does not seem to
anything wrong really with the output (such as in mangled
letters or dropped "fi" and such)

I understand these come out because tex4ht does not have full
support for the fonts. But how come the HTML output looks fine
when looking at it? Or does it mean one simply did not yet encounter
the font problem because the current text being compiled does not
have a text that will show the problem?

I am just trying to decide if I should not use this font when seeing
such warnings, even though the HTML output looks OK.

------------ MWE -------
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{ntheorem}
\newtheorem{theorem}{Theorem}
\usepackage[libertine]{newtxmath}
\DeclareMathOperator{\Res}{Res}
\usepackage[english]{babel}
\usepackage{blindtext}

\begin{document}
\blindtext
\pagestyle{empty}
\begin{theorem}[Residue Theorem]
Let $f$ be analytic in the region $G$ except for the isolated
singularities $a_1,a_2,\dots,a_m$. If $\gamma$ is a closed
rectifiable curve in $G$ which does not pass through any of the
points $a_k$ and if $\gamma\approx 0$ in $G$, then
\[
   \frac{1}{2\pi i}\int_\gamma\! f = \sum_{k=1}^m
   n(\gamma;a_k)\Res(f;a_k)\,.
\]
\end{theorem}
--------------------------------

Compiled using

make4ht --lua -u -c ./nma.cfg -e ./main.mk4 foo5.tex

the files nma.cfg and main.mk4 used in the above command are
in this folder

http://12000.org/tmp/080315/

Using TL 2015

thanks,
--Nasser