<s>
A	O
progressive	B-Application
web	I-Application
application	I-Application
(	O
PWA	B-Application
)	O
,	O
or	O
progressive	B-Application
web	I-Application
app	I-Application
,	O
is	O
a	O
type	O
of	O
application	B-Application
software	I-Application
delivered	O
through	O
the	O
web	O
,	O
built	O
using	O
common	O
web	O
technologies	O
including	O
HTML	B-Language
,	O
CSS	B-Language
,	O
JavaScript	B-Language
,	O
and	O
WebAssembly	B-Language
.	O
</s>
<s>
It	O
is	O
intended	O
to	O
work	O
on	O
any	O
platform	O
with	O
a	O
standards-compliant	O
browser	B-Application
,	O
including	O
desktop	B-Device
and	O
mobile	B-Application
devices	I-Application
.	O
</s>
<s>
Since	O
a	O
progressive	B-Application
web	I-Application
app	I-Application
is	O
a	O
type	O
of	O
webpage	O
or	O
website	O
known	O
as	O
a	O
web	B-Application
application	I-Application
,	O
it	O
does	O
not	O
require	O
separate	O
bundling	O
or	O
distribution	O
.	O
</s>
<s>
Developers	O
can	O
simply	O
publish	O
the	O
web	B-Application
application	I-Application
online	O
,	O
ensure	O
that	O
it	O
meets	O
baseline	O
installation	O
requirements	O
and	O
that	O
users	O
will	O
be	O
able	O
to	O
add	O
the	O
application	O
to	O
their	O
home	B-Operating_System
screen	I-Operating_System
.	O
</s>
<s>
Publishing	O
the	O
app	O
to	O
digital	O
distribution	O
systems	O
like	O
Apple	B-Application
App	I-Application
Store	I-Application
or	O
Google	B-Application
Play	I-Application
is	O
optional	O
.	O
</s>
<s>
As	O
of	O
2021	O
,	O
PWA	B-Application
features	O
are	O
supported	O
to	O
varying	O
degrees	O
by	O
Google	B-Application
Chrome	I-Application
,	O
Apple	B-Application
Safari	I-Application
,	O
Firefox	B-Application
for	I-Application
Android	I-Application
,	O
and	O
Microsoft	B-Application
Edge	I-Application
but	O
not	O
by	O
Firefox	B-Application
for	O
desktop	B-Device
.	O
</s>
<s>
+BrowserSupportComment	O
Windows	B-Application
macOS	B-Application
Linux	B-Application
Android	B-Application
iOS	B-Application
&	O
iPadOS	B-Operating_System
Chromium-based	O
Includes	O
Google	B-Application
Chrome	I-Application
,	O
Microsoft	B-Application
Edge	I-Application
,	O
Brave	B-Application
,	O
Opera	B-Application
,	O
Vivaldi	B-Language
,	O
and	O
others	O
.	O
</s>
<s>
At	O
the	O
launch	O
of	O
the	B-Device
iPhone	I-Device
in	O
2007	O
,	O
Steve	O
Jobs	O
announced	O
that	O
web	B-Application
apps	I-Application
(	O
developed	O
in	O
HTML5	B-Application
using	O
AJAX	B-Device
architecture	O
)	O
would	O
be	O
the	O
standard	O
format	O
for	O
iPhone	B-Device
apps	O
.	O
</s>
<s>
No	O
software	B-Application
development	I-Application
kit	I-Application
(	O
SDK	O
)	O
was	O
required	O
,	O
and	O
the	O
apps	O
would	O
be	O
fully	O
integrated	O
into	O
the	O
device	O
through	O
the	O
Safari	B-Application
browser	I-Application
engine	O
.	O
</s>
<s>
This	O
model	O
was	O
later	O
switched	O
to	O
the	O
App	B-Application
Store	I-Application
,	O
as	O
a	O
means	O
of	O
confounding	O
jailbreakers	B-Application
and	O
appeasing	O
frustrated	O
developers	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
although	O
Apple	O
continued	O
to	O
support	O
web	B-Application
apps	I-Application
,	O
the	O
vast	O
majority	O
of	O
iOS	B-Application
applications	I-Application
shifted	O
toward	O
the	O
App	B-Application
Store	I-Application
.	O
</s>
<s>
Beginning	O
in	O
the	O
early	O
2010s	O
dynamic	O
web	O
pages	O
allowed	O
web	O
technologies	O
to	O
be	O
used	O
to	O
create	O
interactive	O
web	B-Application
applications	I-Application
.	O
</s>
<s>
Responsive	B-General_Concept
web	I-General_Concept
design	I-General_Concept
,	O
and	O
the	O
screen-size	O
flexibility	O
it	O
provides	O
,	O
made	O
PWA	B-Application
development	O
more	O
accessible	O
.	O
</s>
<s>
Continued	O
enhancements	O
to	O
HTML	B-Language
,	O
CSS	B-Language
,	O
and	O
JavaScript	B-Language
allowed	O
web	B-Application
applications	I-Application
to	O
incorporate	O
greater	O
levels	O
of	O
interactivity	O
,	O
making	O
native-like	O
experiences	O
possible	O
on	O
a	O
website	O
.	O
</s>
<s>
In	O
2013	O
,	O
Mozilla	O
released	O
Firefox	B-Operating_System
OS	I-Operating_System
.	O
</s>
<s>
It	O
was	O
intended	O
to	O
be	O
an	O
open-source	B-License
operating	B-General_Concept
system	I-General_Concept
for	O
running	O
web	B-Application
apps	I-Application
as	O
native	O
apps	O
on	O
mobile	B-Application
devices	I-Application
.	O
</s>
<s>
Firefox	B-Operating_System
OS	I-Operating_System
was	O
based	O
on	O
the	O
Gecko	B-Language
rendering	I-Language
engine	I-Language
with	O
a	O
user	O
interface	O
called	O
Gaia	O
,	O
written	O
in	O
HTML5	B-Application
.	O
</s>
<s>
The	O
development	O
of	O
Firefox	B-Operating_System
OS	I-Operating_System
ended	O
in	O
2016	O
,	O
and	O
the	O
project	O
was	O
completely	O
discontinued	O
in	O
2017	O
,	O
although	O
a	O
fork	O
of	O
Firefox	B-Operating_System
OS	I-Operating_System
was	O
used	O
as	O
the	O
basis	O
of	O
KaiOS	B-Device
,	O
a	O
feature	O
phone	O
platform	O
.	O
</s>
<s>
In	O
2015	O
,	O
designer	O
Frances	O
Berriman	O
and	O
Google	B-Application
Chrome	I-Application
engineer	O
Alex	O
Russell	O
coined	O
the	O
term	O
"	O
progressive	B-Application
web	I-Application
apps	I-Application
"	O
to	O
describe	O
apps	O
taking	O
advantage	O
of	O
new	O
features	O
supported	O
by	O
modern	O
browsers	B-Application
,	O
including	O
service	O
workers	O
and	O
web	B-Application
app	I-Application
manifests	O
,	O
that	O
let	O
users	O
upgrade	O
web	B-Application
apps	I-Application
to	O
progressive	B-Application
web	I-Application
applications	I-Application
in	O
their	O
native	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
.	O
</s>
<s>
Google	O
then	O
put	O
significant	O
efforts	O
into	O
promoting	O
PWA	B-Application
development	O
for	O
Android	B-Application
.	O
</s>
<s>
Firefox	B-Application
introduced	O
support	O
for	O
service	O
workers	O
in	O
2016	O
,	O
and	O
Microsoft	B-Application
Edge	I-Application
and	O
Apple	B-Application
Safari	I-Application
followed	O
in	O
2018	O
,	O
making	O
service	O
workers	O
available	O
on	O
all	O
major	O
systems	O
.	O
</s>
<s>
By	O
2019	O
,	O
PWAs	B-Application
were	O
supported	O
by	O
desktop	B-Device
versions	O
of	O
most	O
browsers	B-Application
,	O
including	O
Microsoft	B-Application
Edge	I-Application
(	O
on	O
Windows	B-Application
)	O
and	O
Google	B-Application
Chrome	I-Application
(	O
on	O
Windows	B-Application
,	O
macOS	B-Application
,	O
ChromeOS	B-Operating_System
,	O
and	O
Linux	B-Application
)	O
.	O
</s>
<s>
In	O
December	O
2020	O
,	O
Firefox	B-Application
for	O
desktop	B-Device
abandoned	O
the	O
implementation	O
of	O
PWAs	B-Application
(	O
specifically	O
,	O
removed	O
the	O
prototype	O
"	O
site-specific	B-Protocol
browser	I-Protocol
"	O
configuration	O
that	O
had	O
been	O
available	O
as	O
an	O
experimental	O
feature	O
)	O
.	O
</s>
<s>
A	O
Firefox	B-Application
architect	O
noted	O
:	O
"	O
The	O
signal	O
I	O
hope	O
we	O
are	O
sending	O
is	O
that	O
PWA	B-Application
support	O
is	O
not	O
coming	O
to	O
desktop	B-Device
Firefox	B-Application
anytime	O
soon.	O
"	O
</s>
<s>
Mozilla	O
still	O
plans	O
to	O
support	O
PWAs	B-Application
on	O
Android	B-Application
.	O
</s>
<s>
Since	O
a	O
progressive	B-Application
web	I-Application
app	I-Application
is	O
a	O
type	O
of	O
webpage	O
or	O
website	O
known	O
as	O
a	O
web	B-Application
application	I-Application
,	O
they	O
do	O
not	O
require	O
separate	O
bundling	O
or	O
distribution	O
.	O
</s>
<s>
In	O
particular	O
,	O
there	O
is	O
no	O
requirement	O
for	O
developers	O
or	O
users	O
to	O
install	O
web	B-Application
apps	I-Application
via	O
digital	O
distribution	O
systems	O
like	O
Apple	B-Application
App	I-Application
Store	I-Application
,	O
Google	B-Application
Play	I-Application
,	O
Microsoft	B-Application
Store	I-Application
,	O
or	O
Samsung	B-Application
Galaxy	I-Application
Store	I-Application
.	O
</s>
<s>
To	O
varying	O
degrees	O
,	O
the	O
major	O
app	B-Application
stores	I-Application
support	O
the	O
publication	O
of	O
PWAs	B-Application
.	O
</s>
<s>
Google	B-Application
Play	I-Application
,	O
Microsoft	B-Application
Store	I-Application
,	O
and	O
Samsung	B-Application
Galaxy	I-Application
Store	I-Application
support	O
PWAs	B-Application
,	O
but	O
Apple	B-Application
App	I-Application
Store	I-Application
does	O
not	O
.	O
</s>
<s>
Microsoft	B-Application
Store	I-Application
publishes	O
some	O
qualifying	O
PWA	B-Application
automatically	O
(	O
even	O
without	O
app	O
authors	O
 '	O
request	O
)	O
after	O
discovering	O
them	O
via	O
Bing	B-Application
indexing	O
.	O
</s>
<s>
Progressive	B-Application
web	I-Application
apps	I-Application
are	O
all	O
designed	O
to	O
work	O
on	O
any	O
browser	B-Application
that	O
is	O
compliant	O
with	O
the	O
appropriate	O
web	O
standards	O
.	O
</s>
<s>
Progressive	B-Application
web	I-Application
apps	I-Application
employ	O
the	O
progressive	O
enhancement	O
web	O
development	O
strategy	O
.	O
</s>
<s>
Some	O
progressive	B-Application
web	I-Application
apps	I-Application
use	O
an	O
architectural	O
approach	O
called	O
the	O
App	O
Shell	B-Operating_System
Model	O
.	O
</s>
<s>
In	O
this	O
model	O
,	O
service	O
workers	O
store	O
the	O
Basic	O
User	O
Interface	O
or	O
"	O
shell	B-Operating_System
"	O
of	O
the	O
responsive	B-General_Concept
web	I-General_Concept
design	I-General_Concept
web	B-Application
application	I-Application
in	O
the	O
browser	B-Application
's	O
offline	O
cache	B-General_Concept
.	O
</s>
<s>
This	O
model	O
allows	O
for	O
PWAs	B-Application
to	O
maintain	O
native-like	O
use	O
with	O
or	O
without	O
web	O
connectivity	O
.	O
</s>
<s>
The	O
technical	O
baseline	O
criteria	O
for	O
a	O
site	O
to	O
be	O
considered	O
a	O
progressive	B-Application
web	I-Application
app	I-Application
and	O
therefore	O
being	O
capable	O
of	O
being	O
installed	O
by	O
browsers	B-Application
were	O
described	O
by	O
Russell	O
in	O
a	O
follow-up	O
post	O
and	O
updated	O
since	O
:	O
</s>
<s>
Served	O
over	O
TLS	B-Protocol
and	O
have	O
no	O
active	O
mixed	O
content	O
.	O
</s>
<s>
Progressive	B-Application
web	I-Application
apps	I-Application
must	O
be	O
served	O
via	O
HTTPS	B-Protocol
to	O
ensure	O
user	O
privacy	O
,	O
security	O
,	O
and	O
content	O
authenticity	O
.	O
</s>
<s>
Progressive	B-Application
web	I-Application
apps	I-Application
must	O
use	O
service	O
workers	O
to	O
create	O
programmable	O
content	O
caches	B-General_Concept
.	O
</s>
<s>
Unlike	O
regular	O
HTTP	B-Protocol
web	B-Protocol
cache	I-Protocol
,	O
which	O
caches	B-General_Concept
content	O
after	O
the	O
first	O
use	O
and	O
then	O
relies	O
on	O
various	O
heuristics	O
to	O
guess	O
when	O
content	O
is	O
no	O
longer	O
needed	O
,	O
programmable	O
caches	B-General_Concept
can	O
explicitly	O
prefetch	O
content	O
in	O
advance	O
before	O
it	O
's	O
used	O
for	O
the	O
first	O
time	O
and	O
explicitly	O
discard	O
it	O
when	O
it	O
is	O
no	O
longer	O
needed	O
.	O
</s>
<s>
Reference	O
a	O
web	B-Application
app	I-Application
manifest	O
.	O
</s>
<s>
Information	O
contained	O
in	O
manifest	O
makes	O
PWAs	B-Application
easily	O
shareable	O
via	O
a	O
URL	O
,	O
discoverable	O
by	O
a	O
search	O
engines	O
,	O
and	O
alleviates	O
complex	O
installation	O
procedures	O
(	O
but	O
PWAs	B-Application
still	O
can	O
be	O
listed	O
in	O
a	O
third-party	O
app	B-Application
store	I-Application
)	O
.	O
</s>
<s>
Furthermore	O
,	O
PWAs	B-Application
support	O
native	O
app-style	O
interactions	O
and	O
navigation	O
,	O
including	O
being	O
added	O
to	O
home	B-Operating_System
screen	I-Operating_System
,	O
displaying	O
splash	O
screens	O
,	O
etc	O
.	O
</s>
<s>
In	O
2017	O
,	O
Twitter	B-Application
released	O
Twitter	B-Application
Lite	O
,	O
a	O
PWA	B-Application
alternative	O
to	O
the	O
official	O
native	O
Android	B-Application
and	O
iOS	B-Application
apps	I-Application
.	O
</s>
<s>
According	O
to	O
Twitter	B-Application
,	O
Twitter	B-Application
Lite	O
consumed	O
only	O
1-3	O
%	O
of	O
the	O
size	O
of	O
the	O
native	O
apps	O
.	O
</s>
<s>
Starbucks	B-Application
provides	O
a	O
PWA	B-Application
that	O
is	O
99.84	O
%	O
smaller	O
than	O
its	O
equivalent	O
iOS	B-Application
app	I-Application
.	O
</s>
<s>
After	O
deploying	O
its	O
PWA	B-Application
,	O
Starbucks	B-Application
doubled	O
the	O
number	O
of	O
online	O
orders	O
,	O
with	O
desktop	B-Device
users	O
ordering	O
at	O
about	O
the	O
same	O
rate	O
as	O
mobile	O
app	O
users	O
.	O
</s>
<s>
A	O
2018	O
review	O
published	O
by	O
Forbes	O
,	O
found	O
that	O
users	O
of	O
Pinterest	B-Application
's	O
PWA	B-Application
spent	O
40%	O
more	O
time	O
on	O
the	O
site	O
compared	O
to	O
the	O
previous	O
mobile	O
website	O
.	O
</s>
<s>
Flipkart	O
saw	O
60%	O
of	O
customers	O
who	O
had	O
uninstalled	O
their	O
native	O
app	O
return	O
to	O
use	O
the	O
Flipkart	O
PWA	B-Application
.	O
</s>
<s>
Lancôme	O
saw	O
an	O
84%	O
decrease	O
in	O
time	O
until	O
the	O
page	O
is	O
interactive	O
,	O
leading	O
to	O
a	O
17%	O
increase	O
in	O
conversions	O
and	O
a	O
53%	O
increase	O
in	O
mobile	O
sessions	O
on	O
iOS	B-Application
with	O
their	O
PWA	B-Application
.	O
</s>
<s>
There	O
are	O
many	O
technologies	O
commonly	O
used	O
to	O
create	O
progressive	B-Application
web	I-Application
apps	I-Application
.	O
</s>
<s>
A	O
web	B-Application
application	I-Application
is	O
considered	O
a	O
PWA	B-Application
if	O
it	O
satisfies	O
the	O
installation	O
criteria	O
and	O
thus	O
can	O
work	O
offline	O
and	O
can	O
be	O
added	O
to	O
device	O
home	B-Operating_System
screen	I-Operating_System
.	O
</s>
<s>
To	O
meet	O
this	O
definition	O
,	O
all	O
PWAs	B-Application
require	O
at	O
minimum	O
a	O
service	O
worker	O
and	O
a	O
manifest	O
.	O
</s>
<s>
The	O
web	B-Application
app	I-Application
manifest	O
is	O
a	O
W3C	O
specification	O
defining	O
a	O
JSON-based	O
manifest	O
(	O
usually	O
labelled	O
manifest.json	O
)	O
to	O
provide	O
developers	O
a	O
centralized	O
place	O
to	O
put	O
metadata	O
associated	O
with	O
a	O
web	B-Application
application	I-Application
including	O
:	O
</s>
<s>
This	O
metadata	O
is	O
crucial	O
for	O
an	O
app	O
to	O
be	O
added	O
to	O
a	O
home	B-Operating_System
screen	I-Operating_System
or	O
otherwise	O
listed	O
alongside	O
native	O
apps	O
.	O
</s>
<s>
iOS	B-Application
Safari	B-Application
partially	O
implements	O
manifests	O
,	O
while	O
most	O
of	O
the	O
PWA	B-Application
metadata	O
can	O
be	O
defined	O
via	O
Apple-specific	O
extensions	O
to	O
the	O
meta	O
tags	O
.	O
</s>
<s>
WebAssembly	B-Language
allows	O
precompiled	B-Application
code	O
to	O
run	O
in	O
a	O
web	B-Application
browser	I-Application
,	O
at	O
near-native	O
speed	O
.	O
</s>
<s>
Thus	O
,	O
libraries	O
written	O
in	O
languages	O
such	O
as	O
C	B-Language
can	O
be	O
added	O
to	O
web	B-Application
apps	I-Application
.	O
</s>
<s>
Due	O
to	O
the	O
cost	O
of	O
passing	O
data	O
from	O
JavaScript	B-Language
to	O
WebAssembly	B-Language
,	O
near-term	O
uses	O
will	O
be	O
mainly	O
number-crunching	O
(	O
such	O
as	O
voice	O
recognition	O
and	O
computer	O
vision	O
)	O
,	O
rather	O
than	O
whole	O
applications	O
.	O
</s>
<s>
Web	O
Storage	O
is	O
a	O
W3C	O
standard	O
API	O
that	O
enables	O
key-value	O
storage	O
in	O
modern	O
browsers	B-Application
.	O
</s>
<s>
The	O
API	O
consists	O
of	O
two	O
objects	O
,	O
sessionStorage	O
(	O
that	O
enables	O
session-only	O
storage	O
that	O
gets	O
wiped	O
upon	O
browser	B-Application
session	O
end	O
)	O
and	O
localStorage	O
(	O
that	O
enables	O
storage	O
that	O
persists	O
across	O
sessions	O
)	O
.	O
</s>
<s>
Indexed	O
Database	O
API	O
is	O
a	O
W3C	O
standard	O
database	O
API	O
available	O
in	O
all	O
major	O
browsers	B-Application
.	O
</s>
<s>
The	O
API	O
is	O
supported	O
by	O
modern	O
browsers	B-Application
and	O
enables	O
storage	O
of	O
JSON	B-General_Concept
objects	O
and	O
any	O
structures	O
representable	O
as	O
a	O
string	O
.	O
</s>
<s>
It	O
is	O
able	O
to	O
check	O
the	O
availability	O
of	O
a	O
remote	O
server	O
and	O
to	O
cache	B-General_Concept
content	O
when	O
that	O
server	O
is	O
available	O
,	O
and	O
serve	O
that	O
content	O
later	O
to	O
the	O
document	O
.	O
</s>
<s>
Service	O
workers	O
can	O
handle	O
push	B-General_Concept
notifications	I-General_Concept
and	O
synchronize	O
data	O
in	O
the	O
background	O
,	O
cache	B-General_Concept
or	O
retrieve	O
resource	O
requests	O
,	O
intercept	O
network	O
requests	O
and	O
receive	O
centralized	O
updates	O
independently	O
of	O
the	O
document	O
that	O
registered	O
them	O
,	O
even	O
when	O
that	O
document	O
is	O
not	O
loaded	O
.	O
</s>
<s>
Registration	O
involves	O
telling	O
the	O
browser	B-Application
the	O
location	O
of	O
the	O
service	O
worker	O
in	O
preparation	O
for	O
installation	O
.	O
</s>
<s>
Installation	O
occurs	O
when	O
there	O
is	O
no	O
service	O
worker	O
installed	O
in	O
the	O
browser	B-Application
for	O
the	O
web	B-Application
app	I-Application
,	O
or	O
if	O
there	O
is	O
an	O
update	O
to	O
the	O
service	O
worker	O
.	O
</s>
<s>
Activation	O
occurs	O
when	O
all	O
of	O
the	O
PWA	B-Application
's	O
pages	O
are	O
closed	O
,	O
so	O
that	O
there	O
is	O
no	O
conflict	O
between	O
the	O
previous	O
version	O
and	O
the	O
updated	O
one	O
.	O
</s>
