Dunno whether this is perhaps already known and not considered a bug.
Dunno whether, if it is considered a bug, it is seen as an Emacs bug,
since Emacs manuals are not the only ones exhibiting the bug.

Anyway, If I look at the GNU manuals online, such as this one:
http://www.gnu.org/software/emacs/manual/elisp.html, and I choose "HTML
- with one web page per node.", and I start visiting nodes, I see that
the individual lines of text are generally centered, instead of being
left-justified.

This looks awful and makes reading more difficult.  A list of bullets,
for instance, does not have the bullets aligned.  Here, as an example,
is the first node of the Elisp manual:
http://www.gnu.org/software/emacs/manual/html_node/elisp/Introduction.html#Introduction

And this is what it looks like - see attachment.

In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2014-01-07 on ODIEONE
Bzr revision: 115916 bzg@gnu.org-20140107233629-du2solx6tmxnx0np
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/binary --enable-checking=yes,glyphs
 'CFLAGS=-O0 -g3' LDFLAGS=-Lc:/Devel/emacs/lib
 CPPFLAGS=-Ic:/Devel/emacs/include'