Support for XHTML and CSS is limited and often quirky. Scripting can be considered to be not available. Keep everything as simple as possible.

Think book, not website!

Images:

Keep images relatively small and don't insert them in tables. It is a good idea to use the Zoom is proportional to document width property in the Insert Image dialog to size the image automatically relative to the eBook viewer page.

Videos:

Videos are only supported on iOS, and will not work in eBooks on any other platform. Apple recommends a maximum of not more than 11 megabytes of images per chapter for acceptable performance, particularly on the iPad 1. Images with more than two million pixels (width x height) are not guaranteed to display in iBooks®, and they would also be much too big to display normally on any eBook reader, including relatively large readers like the iPad..

If you want to show a large video put it on YouTube and embed it with the YouTube option, then the iPad will switch to the YouTube app automatically and open the video there. Videos will not work on any other ePub reader.

Remember that most eBook readers have a very narrow effective page width, so if you want images to be visible they should be placed directly on the page in paragraphs of their own. They should also not be too high, because tall images will force page breaks and leave ugly gaps in your pages.

Generally, it is better to avoid images in  tables entirely. It is much better to put every image directly on the page in a paragraph of its own. It is much easier for an image in a table to break your layout than an image on the page. This applies particularly to tables with more than one column.

Topics designed for generic ePub and Kindle ePub have the following requirements and restrictions:

•Regular text is OK.

•Formatted text is OK.

•Font management requires special consideration – see Managing Fonts.

•Internal topic links only – no file links or script links.

•External HTTP links are supported but they won't work on hardware readers without web access!

•All image formats supported in H+M can be used, images are converted according to your HTML Export options settings if possible, but conformity with eBooks requirements have priority.

•Simple tables only. If possible, avoid tables entirely. No nested tables, no fixed-width tables, no complex tables, no tables with background images.

•No JavaScript, no manual page breaks.

•Video is only supported in Apple iBooks®. Consider it to be not available in any other reader, so only include videos if you in eBooks designed exclusively for iBooks.

•Hotspots in images are not supported by any ePub readers.

The TOC in generic ePub and Kindle ePub has the following restrictions and requirements:

•Normal topics entries and chapter entries that are topics (chapters with text) are OK.

•Topic entries with anchor references are OK (a TOC entry that links to an anchor in a topic).

•TOC entries that are chapters without text are not recommended. The TOC entry will point to the first topic of the chapter.

•No TOC entries that link to external help files or web pages.

These eBooks are special compressed formats with internal file systems taht only support 7-bit characters in the internal file names. Make sure that your topic IDs only use a..z, A..Z, 0..9 and _, no other characters. This is also general good practice to avoid possible problems in all project types.

This requirement is the same as in PDF. Only topics that have TOC entries will be included in your eBook output. All topics without TOC entries are ignored and will not be included in your eBook.