I created a reverse proxy on an opengrok application with relative easy using the mod_proxy_html package. That was until I realized the parser was stripping out attributes used by the opengrok server to do things like 'show more/less' content. When I turned trace on, I see things like this 'AH01418: Bogus HTML attribute data-toggle-state of span dropped' during the page rendering. Having a directive to tell the SAX parser to ignore the custom tag attributes listed would work perfectly. e.g. ProxyHTMLIgnoreAttribute data-toggle-state data-priority ... I did a search and couldn't find a satisfactory workaround. Setting 'ProxyHTMLDocType HTML Legacacy' does horrible things to the page styling. As it is, I will have to scrap mod_proxy_html and rewrite the url's as I hit them.
The data-* attribute is used to store custom data private to the page or application. The data-* attribute gives us the ability to embed custom data attributes on all HTML elements. It seems that the data-* custom attribute is allowed for all HTML tags. You are removing them.