This site browses the 4927 emails from Really Good Emails to count the use of certain HTML and CSS features. The numbers on this page indicate the percentage of HTML emails that use the corresponding HTML or CSS feature at least once.
Data is updated every month.

Metas Percentage
viewport
81.53034%
content-type
71.90988%
x-ua-compatible
48.56911%
format-detection
19.13944%
x-apple-disable-message-reformatting
0.10148%
robots
9.64076%
referrer
4.4043%
twitter:title
2.41526%
generator
2.2326%
twitter:description
1.86726%
twitter:card
1.60341%
twitter:image:src
1.31926%
description
1.27867%
filler
1.13659%
apple-mobile-web-app-capable
0.79156%
apple-mobile-web-app-status-bar-style
0.548%
author
0.5277%
apple-touch-fullscreen
0.50741%
x-apple-disable-message-reformating
0.30444%
handheldfriendly
0.30444%
content-language
0.30444%
twitter:image
0.24356%
title
0.24356%
color-scheme
0.22326%
language
0.20296%
cache-control
0.20296%
twitter:site
0.18267%
mobileoptimized
0.14207%
supported-color-schemes
0.12178%
keywords
0.12178%
progid
0.12178%
google
0.08119%
subject
0.08119%
sg:flowtag
0.08119%
sg:pagetag
0.08119%
content-style-type
0.08119%
pragma
0.08119%
googlebot
0.08119%
templatename
0.06089%
theme-color
0.04059%
expires
0.04059%
msapplication-config
0.04059%
twitter:creator
0.04059%
responsive
0.04059%
unsubscribe_url
0.04059%
contenpt-type
0.04059%
twitter:image:width
0.0203%
twitter:image:height
0.0203%
twitter:url
0.0203%
apple-mobile-web-app-title
0.0203%
content
0.0203%
content-version
0.0203%
mktem
0.0203%
email-subject
0.0203%
msapplication-tilecolor
0.0203%
type
0.0203%
focus
0.0203%
frequency
0.0203%
start_date
0.0203%
csrf-token
0.0203%
csrf-param
0.0203%
coantent-type
0.0203%
content=
0.0203%
5px top spacer
0.0203%
version
0.0203%
destiny
0.0203%
campaign_name
0.0203%
no-cache
0.0203%
google-site-verification
0.0203%
p:domain_verify
0.0203%
#iewport
0.0203%
campaign_code
0.0203%
edata-campaignid
0.0203%
edata-mailclass
0.0203%
ternair-paste-as-plaintext
0.0203%
shared email widget
0.0203%
2017-11-21-aus-d-1-type
0.0203%
jobnumber
0.0203%
image alt text
0.0203%
url
0.0203%
divider color
0.0203%
content- type
0.0203%
copy-type
0.0203%
from
0.0203%
content-security-policy
0.0203%