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Plain-Text vs HTML E-Mail

Sizes and Errors: 2021 Analysis

Copyright © 2019-2021 by David E. Ross

Definitions

bloat: the increase in the size of an HTML-formatted message to convey the same textual content as a plain-text message
bloat factor: a measure of bloat, computed by dividing the size of the HTML-formatted message by the size of the plain-text message that has the same content. If a 3,000 byte HTML-formatted message has the same content as a 600 byte plain-text message, the bloat factor is 3,000 ÷ 600 = 5.0.
KB: kilobyte, 1,000 bytes

History

In the fall of 2008, I examined 20 HTML-formatted E-mail messages. From this study, I concluded that the average bloat factor was 3.4 and that HTML-formatted messages contained an average of 7.4 HTML errors per KB.
Early in 2010, I decided to repeat this study for several reasons.
- I did not save the messages from the earlier study in case I wanted to re-examine them.
- I could not recall whether I made sure that I did not use more than one message from the same sender.
- I computed the average bloat by taking the total size of all HTML-formatted messages and divided by the total size of the equivalent plain-text messages. I was curious what the result would have been if I had averaged the individual bloats of each message.
- I suspected that, since the last study, improvements in E-mail clients might have resulted in less bloat and fewer HTML errors in HTML-formatted messages.
I again collected 20 HTML-formatted E-mail messages. This time, the average bloat factor was 4.6 (worse than in 2008); and HTML-formatted messages contained an average of 4.6 HTML errors per KB (less than the ratio in 2008).
After two more years during which E-mail clients might have further evolved, I repeated the study with a fresh set of 20 HTML-formatted messages collected during January 2012. Unlike the 2010 study, however, I did not collect data on what E-mail clients were used; in 2010, half the messages failed to indicate any client while in 2012 that grew to three-fourths of the message not indicating any client, preventing any use of client identifications. The average bloat for HTML-formatted messages was 4.0 times the size of equivalent plain-text messages, with 5.0 HTML errors per KB.
I repeated the study with another 20 E-mail messages collected in June of 2015 to see what changes might have occurred in HTML-formatted E-mail in the prior three years. The average bloat factor for HTML-formatted messages increased to 10.0 times the size of the equivalent plain-text content, with 6.1 HTML errors per KB.
I repeated the study with another 20 E-mail messages collected in June of 2019 to see what changes might have occurred in HTML-formatted E-mail in the prior four years. The average bloat factor for HTML-formatted messages increased to 16.0 times the size of the equivalent plain-text content, with 7.3 HTML errors per KB.

In September 2021, I repeated the study, again with 20 HTML-formatted and multi-part (plain-text and HTML-formatted combined) messages each from a different sender.

Findings

The average bloat factor for HTML-formatted messages was 15.5 times the size of the equivalent plain-text content. The range was from 3.1 to 38.3. The largest HTML-formatted message had a bloat factor of 18.3. The smallest had 6.4, which was not the smallest bloat factor. The bloat factors of the messages were not correlated with their sizes.
HTML-formatted messages contain an average of 10.0 HTML syntax errors per KB of HTML markup. The range was from 0.1 to 47.6 errors per KB. The largest HTML-formatted message had a total of 20 HTML errors (0.1 errors per KB). The smallest had 73 (47.6 errors per KB). As with bloat, there was no correlation between message size and the number of HTML errors.
There were no significant correlations between bloat factors and the proportion of HTML errors.

Conclusions

HTML-formatted messages are excessively larger than plain-text messages conveying the same information. Improvements in E-mail clients has not resulted in reductions in bloat. Bloat means longer download time for those who still use dial-up connections to the Internet and slower anti-virus checking for everyone. Bloat also requires more disc space to store messages for both users and mail servers; this can impact businesses that are required by law to archive E-mail messages. Mass-mailings of HTML-formatted messages may cause Internet congestion by consuming excessive bandwidth.
A significant source of bloat is the nesting of unnecessary <div>—</div> elements. I examined a message with a bloat factor of over 17. It had the following markup 19 times where one would have sufficed:
<div class="moz-forward-container">
Improvements in E-mail clients since 2012 have not resulted in any reduction in HTML errors. Indeed, the proportion of HTML errors relative to the size of E-mail messages has increased. Such invalid HTML may cause garbled quoting of a message when replying to it or forwarding it using an E-mail client different from the client that originally sent the message. It can also prevent processing a message through an audio application for a blind user; for a business or government agency, this might be a violation of the Americans with Disabilities Act (ADA).
One problem that might not appear as an HTML error is excessive nesting of tables. I examined a message that had seven tables, not separate tables but each nested within another. This is considered very poor HTML design.

Methodology

I collected 20 messages that were either HTML-formatted or were two-part (containing both plain text and HTML). Making sure that no two messages were from the same source, I stored each message four times. I excluded any attachments (which, for HTML-formatted messages, means excluding any images or background). The first two of these were used in my analysis. The last two were saved in case I needed further information.

I stored the readable content from the first line to the end of the message into a plain-text file.
I stored the raw message (the source file) from the <x-html> tag to the </x-html> tag (excluding those tags) into an HTML file.
I saved the source file, which in some cases reflected a 2-part message — a message that included both plain-text and HTML formatting. This included the entire header section.
I stored the source file without the header section.

I recorded the sizes of the first two in a spread sheet — the first file as Plain and the second as HTML. Dividing the total of the HTML sizes by the total of the Plain sizes gave me the average bloat factor. Because bloat is meaningful in the gross context over many messages — in terms of bandwidth impacts, disc space occupied, etc — this average is based on the total size of all messages compared with the total size of the equivalent plain-text content. If I had averaged the individual bloat factors of each message, the result would have been 16.9, greater than the 16.0 reported under "Findings".

Note that the Plain files might not contain all the content intended for the message. This would be caused by placing text within images, which would make the message incomplete for a blind person using an audio browser. For commercial messages, this would be a violation of the Americans with Disabilities Act.

For HTML errors, I used the W3C Markup Validation Service. I input the content of an HTML file. If the message's HTML included a <!DOCTYPE> declaration (which is required for Web pages), I selected the "Validate Full Document" option. Otherwise, I selected the "Validate HTML fragment" option, specifying "HTML 4.01 Transitional" (which is the least restrictive HTML 4.01 syntax) and then repeated with "HTML5 (experimental)", choosing the least number of errors between them. I recorded the number of errors on the same spreadsheet. Dividing the number of errors by the HTML size and multiplying by 1,000 gave me the proportion of HTML errors per KB of HTML for each message. Because the impact of HTML errors falls on individual messages, I then took the average of the individual proportions. If I had instead used the total number of errors versus the total HTML sizes, the result would have been 6.4 HTML errors per KB of message size, fewer than the 7.3 reported under "Findings".

NOTE WELL: In the chart below, two-part messages are indicated. The total size of a two-part message is equal to the sum of the plain-text and HTML-formatted sizes plus about 4-6 KB for the header section. Thus, two-part messages might appear to increase bloat. This was not considered during any of these studies, however, because part of that increase — the plain-text part — is actually valid text content. Nevertheless, two-part messages require more bandwidth to send and receive and more disc space to store than messages that are only HTML-formatted.

Raw Data
(sizes in bytes)
Msg #	Plain Size	HTML Size	Bloat Factor	HTML Errors	Errors per KB	2-Part Message
1	3,339	51,501	15.4	21	0.4
2	8,159	149,673	18.3	20	0.1
3	1,457	41,767	28.7	145	3.5
4	2,215	9,233	4.2	277	30.0
5	441	2,678	6.1	48	17.9
6	2,122	35,399	16.7	561	15.8	x
7	324	1,976	6.1	23	11.6	x
8	3,306	52,671	15.9	829	15.7	x
9	768	10,210	13.3	38	3.7
10	1,697	37,737	22.2	864	22.9
11	919	15,998	17.4	57	3.6	x
12	3,575	45,933	12.8	495	10.8	x
13	977	3,004	3.1	6	2.0
14	1,345	26,629	19.8	89	3.3	x
15	1,705	55,694	32.7	274	4.9	x
16	3,045	19,589	6.4	22	1.1	x
17	1,327	50,842	38.3	28	0.6
18	5,646	56,783	10.1	212	3.7	x
19	241	1,533	6.4	73	47.6	x
20	4,195	54,494	13.0	69	1.3	x

New study 24 September 2021