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The tension among recent early- and late-Universe determinations of Hubble.s constant (H0) is one of the potentially most consequential open problems in astrophysics. Specifically comma the present-day measurement based on a well calibrated empirical distance ladder differs by 8.3 +-- 2.0% (at 4.2 sigma) from the cosmology-dependent H0 value derived from the Planck mission.s observations of the Cosmic Microwave Background. It is now crucial to demonstrate the accuracy of the distance ladder by scrutinizing even presumably small systematic errors to either resolve or intensify the tension comma whereby the latter could place fundamental physics at the verge of a breakthrough. We propose to exploit HST-WFC3.s unique UV capabilities to validate and improve the late-Universe H0 measurement based on Cepheid variable stars and type-Ia supernovae (SNeIa). WFC3-UVIS F300X observations of Cepheids in the crucial anchor galaxy NGC4258 will quantify the occurrence of Cepheids in or near their natal (cluster) environments. This will allow us to improve the stellar association bias correction currently applied to the distance ladder by a) comparing clustering frequency comma type comma and magnitude among all distance ladder anchor galaxies (NGC4258 comma Milky Way comma LMC) b) comparing Cepheid clustering properties between NGC4258 and the SNeIa host M101 under nearly identical observing conditions c) increasing clustering statistics to investigate their dependence on Cepheid properties. Coordinated parallel observations of a new halo field will accurately calibrate the tip of the red giant branch (TRGB) luminosity and improve the TRGB.s ability to serve as a Cepheid-independent first distance ladder rung.