font-size:inherit break rt tag

[yumetodo]

「・・・<ruby>本気<rt>ほんき</rt></ruby>で？」  
「<ruby>本気<rt>マジ</rt></ruby>で」

when I wrote such markdown and build gitbook,

font-size of <rt> become very big!(the font-size of ほんき is same as the font-size of 本気)

The result I expected is

.markdown-section * {
	box-sizing:border-box;
	-webkit-box-sizing:border-box;
	font-size:inherit
}

font-size:inherit cause this. However, I cannot detect which less file produce this line.

$ gitbook --version
CLI version: 2.3.0
GitBook version: 3.2.2
$ node -v
v7.0.0
$ npm --version
4.0.2
$ uname -a
Linux yumetodo-desktop 4.4.0-43-generic #63-Ubuntu SMP Wed Oct 12 13:48:03 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

firefox: 49.0.2