[jhowell]

I did some playing around with this, adding the style "line-height: 2.5em" to a <br> element. I used the Kindle Previewer to convert the book in order to avoid any issues that may be introduced by calibre conversion.

In the MOBI portion of the Master MOBI created by the Previewer, the style is simply eliminated from the <br>.

In the KF8 (azw3) the style is left intact, but is ignored by the renderer when displaying the book.

In the KFX (enhanced typesetting) view created by the Previewer the line-height was retained, but applied to the line above the break, causing there to be two gaps in the paragraph. This reflects how most modern Kindle devices and apps would display that markup in a book sold on Amazon.

I recommend just using multiple <br> elements and living with the restriction of having only integer multiple sizes available.