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There are some formatting which are not support by normal non css html like for example, indent and text background color. In this case, does RV has such a feature to allow save mix html. Example when I call Rv.SaveHTML, the exported HTML will be in normal non css html, but for some those formatting that i mentioned above like indent, it will be saved as inline css.

Thanks

Sorry, there is no such option, and it's not planned.
The only possibility is using CSS for non-text items, include rvsoForceNonTextCSS in the Options parameter of SaveHTML.