  The Zwicky Transient Facility (ZTF) reported the event "ZTF19abanrhr" as a
candidate electromagnetic (EM) counterpart at a redshift $z=0.438$ to the
gravitational wave (GW) emission from the binary black hole merger GW190521.
Assuming that ZTF19abanrhr is the {\it bona fide} EM counterpart to GW190521,
and using the GW luminosity distance estimate from three different waveforms
NRSur7dq4, SEOBNRv4PHM, and IMRPhenomPv3HM, we report a measurement of the
Hubble constant $H_0= 50.4_{-19.5}^{+28.1}$ km/s/Mpc, $ 62.2_{-19.7}^{+29.5}$
km/s/Mpc, and $ 43.1_{-11.4}^{+24.6}$ km/s/Mpc (median along with $68\%$
credible interval) respectively after marginalizing over matter density
$\Omega_m$ (or dark energy equation of state $w_0$) assuming the flat LCDM (or
wCDM) model. Combining our results with the binary neutron star event GW170817
with its redshift measurement alone, as well as with its inclination angle
inferred from Very Large Baseline Interferometry (VLBI), we find $H_0=
67.6_{-4.2}^{+4.3}$ km/s/Mpc, $\Omega_m= 0.47_{-0.27}^{+0.34}$, and $w_0=
-1.17_{-0.57}^{+0.68}$ (median along with $68\%$ credible interval) providing
the most stringent measurement on $H_0$ and the first estimation on $\Omega_m$
and $w_0$ from bright standard siren. In the future, $1.3\%$ measurement of
$H_0=68$ km/s/Mpc and $28\%$ measurement of $w_0=-1$ is possible from about
$200$ GW190521-like sources.

Cosmology and Nongalactic Astrophysics;High Energy Astrophysical Phenomena;General Relativity and Quantum Cosmology