[davidfor]

Originally Posted by FrustratedReader

That's not clear anywhere. It seemed like a driver for later Kobos like Aura HD and Kobo H2O.

I have no idea where you got that idea. Reading the first post of the thread for the KoboTouchExtended driver will tell you what it does. Posting, "Do I need this?", like a lot of people have, would have told you.

I definitely do not have "kobo_extra.css"

In that case, I don't know why you turned the option on.

At any rate with these two options off, the Kobo layout now matches the screen in LibreOffice Writer when I create a copy of the source with small page size, 5pt margins, no headers and no footers and page numbers / ... removed from Contents by edit. Also matches Aldiko, Bluefire, (Android tablet & phone) Calibre eReader app, Binatone Readme Daily (Only in Adobe folder. Has LCD screen!). Nook Simple Touch and Sony PRS350. Also PW3 and later Basic Kindle when AZW and recent FW is used.
I used Calibre on Windows XP for a couple of years with three models of Kindle (Paperwhite, Basic touch & DXG) as well as apps on various Android things. Then I got the Kobo original H2O hoping at about 266 DPI and 6.8" screen it would be better for PDFs than the DXG (it's maybe slightly better, but still rubbish for magazine, Letter or A4 PDFs). At first it did as I expected with formatting.
Then the bug on paragraph indent appeared. By then I was using Calibre on Linux. I'd assumed it was a Firmware update that caused it. I'd not changed anything in Calibre for ages till the problem appeared. The only solution I'd found was to use "ems". Both Inches and Points came out haif as wide on the Kobo H2O compared to anything else.

And my understanding is that using non-absolute measurements is generally recommended for epubs. And those hard-coded 5pt margins? One of the reasons for the "Modify CSS" option.

Perhaps at some stage the Kobo Driver was updated. Or I changed the CSS setting, or I'd changed from one Kobo driver to another.

None of the default options in the KoboTouch or KoboTouchExtended drivers have changed. And the options you are talking about have been in the drivers a long time. Both the driver and the Modify CSS function were around a long time before the Aura H2O was released. The extended driver option defaults to on (pointless otherwise), the Modify CSS defaults to off. That has never changed.

However if the settings in the driver are clearly labelled, both with clearer on dialogue text and the mouse hover text I'd have been spared a lot of wasted time and grief.

Well the "Modify CSS" option is clearly labelled as you demonstrated above. The option in the extended driver possibly isn't. But, it is when you understand the purpose of the driver.

I've not tested to see which of the two settings I unchecked has fixed it. Obviously for what I want (Seeing format of same actual eBook on different devices as it would be from Amazon & Smashwords) both settings need unchecked.
If I'm wanting to waste time I may experiment to see if either setting "on" on its own causes the indent to be half width.
I'd never have imagined the eBook edit / CSS feature on the Calibre ePub v2 conversion from odt file or mobi file was showing DIFFERENT css/epub to what was being sent to the Kobo! A Driver should only transfer the file and update eReader database metadata and/or covers if needed. It should NEVER do a file content conversion or edit CSS. I'd never have suspected such a thing.

The job of the driver is to get the books on the device ready to be read. That means the books might need to be manipulated in different ways depending on the device. Or, the features people want to use. And these two functions have been explicitly written to make these changes. And are well documented. Both functions were the result of many requests from people here. Many, many requests.

And of course, the send-to-device process will do a conversion if there is no suitable format for the device. That is specified in the driver. So, the extended driver or the Modify CSS function is just an extension of this.