Does this keep my KML styling (icons, colors, line width)?

No - only geometry and placemark properties (name, description, ExtendedData) carry over, not KML style definitions.

Does this handle nested KML folders?

Yes - every Placemark is read regardless of which Folder/Document nests it.

Convert a KML file (e.g. from Google Earth) to GeoJSON, in your browser.

Input (KML)

Output (GeoJSON)

KML/GPX parsing only runs in the browser.

Runs entirely in your browser - nothing is sent to a server.

Does this keep my KML styling (icons, colors, line width)?: No - only geometry and placemark properties (name, description, ExtendedData) carry over, not KML style definitions.
Does this handle nested KML folders?: Yes - every Placemark is read regardless of which Folder/Document nests it.