Using “AozoraEpub3,” a tool for converting text files into EPUBs, to create vertical electronic books and transfer\/view them on Kindle devices via the Send-to-Kindle feature has become a standard choice for self-publishing e-books.<\/p>\n
The key to achieving a dense page layout similar to a physical paperback book and improving the reading experience in this environment is adjusting the line spacing (line-height) via CSS. In Kindle’s default display, line spacing is set wide (equivalent to approx. 1.8em), which tends to result in fewer characters per screen.<\/p>\n
The purpose of this initiative was to customize the CSS (vertical.css) within AozoraEpub3 and intentionally narrow the line spacing to reduce the frequency of page turns and increase viewing efficiency on the device. Specifically, the quantitative goal was to establish a display density that increases the number of displayed characters per screen by about 30% or more by setting a narrow line spacing of around 1.1em, down from the standard 1.8em.<\/p>\n
However, entering July 2026, when sending EPUB files applying CSS line-spacing adjustments that had previously functioned normally via Send-to-Kindle, an issue occurred where the settings were not reflected on the device at all.<\/p>\n
Specifically, despite making narrow specifications such as line-height: 0.8em or 1.10em on the CSS side, opening the file on the Kindle forcibly resets it to the standard wide line spacing.<\/p>\n
Even after repeatedly modifying AozoraEpub3’s template CSS description, regenerating, and sending the EPUB, no changes whatsoever were seen in the display on the device, resulting in the settings being invalidated.<\/p>\n
To isolate whether the cause lay in the local conversion settings or the transfer environment, comparative verification using past data was conducted.<\/p>\n
As a verification benchmark, checking a vertical EPUB file transferred via Send-to-Kindle on April 21, 2026, confirmed that at that time, narrow line spacing specifications (1.13em or less) were normally reflected and displayed with the intended character density.<\/p>\n
Therefore, we conducted an experiment of resending the exact same EPUB file\u2014generated back on April 21, 2026, and confirmed to display normally\u2014via Send-to-Kindle once again without performing any local re-conversion or editing.<\/p>\n
As a result of the experiment, even though it was the same file that displayed normally back in April, resending it in July confirmed that the line-height settings were invalidated and reset to wide line spacing.<\/p>\n
From these results, it became certain that the issue was not due to AozoraEpub3’s output specifications or local environmental issues, but rather that Amazon’s cloud conversion engine (the EPUB-to-KFX conversion pipeline in Send-to-Kindle) changed its validation specifications for vertical CSS sometime between late April and July 2026.<\/p>\n
To identify up to what numerical value line-height specifications are permitted in Amazon’s server conversion, we created test EPUBs with gradually changing line-height values and repeated transfer experiments.<\/p>\n
The verification results are as follows:<\/p>\n
\u30fbline-height: 80% \/ 0.8em \u2192 Invalidated (Reset to standard line spacing)
\n\u30fbline-height: 100% \/ 1.0em \u2192 Invalidated (Reset to standard line spacing)
\n\u30fbline-height: 110% \/ 1.10em \u2192 Invalidated (Reset to standard line spacing)
\n\u30fbline-height: 112% \/ 1.12em \u2192 Invalidated (Reset to standard line spacing)
\n\u30fbline-height: 113% \/ 1.13em \u2192 Invalidated (Reset to standard line spacing)
\n\u30fbline-height: 114% \/ 1.14em \u2192 Normally reflected (Applied with condensed line spacing)<\/p>\n
As a result of the experiments, it was found that the Send-to-Kindle cloud conversion engine establishes a threshold that automatically deletes and resets specifications of 1.13em (113%) or less from the perspective of “preventing layout collapse.”<\/p>\n
Currently, by specifying 1.14em for the body element in AozoraEpub3’s \/aozoraepub3\/template\/OPS\/css\/vertical.css as follows, you can bypass the filter and maintain the minimum applicable line spacing.<\/p>\n
\n@charset "utf-8";\n@namespace "http:\/\/www.w3.org\/1999\/xhtml";\n\nbody {\nline-height: 1.14em !important;\n}\n<\/pre>\n <\/p>\n
Future Verification Policy and Inquiry Status with Amazon<\/h2>\n
At this stage, specifying line-height: 1.14em !important; is the limit value (provisional solution) for line spacing that can be applied via Send-to-Kindle.<\/p>\n
However, a complete workaround (CSS syntax or HTML structure hacks) to revert to a higher-density layout like 1.10em or 1.00em as was possible before April 2026 has not yet been identified.<\/p>\n
We plan to continue investigating countermeasures.<\/p>\n