What does the rule about a fixed font?

In one old article about SEO optimization read the rule:
It is impossible to design a fixed size font of the main text

As in the subject of design, layout and SEO are not particularly whisking, I would like to ask the guru what does it mean? Type need to write:
body {
font-size: 120%;
}

not
body {
font-size: 16 px;
}

?
And again, how this rule is important at the present time?
Thanks in advance to all the answers.
July 12th 19 at 16:49
2 answers
July 12th 19 at 16:51
Solution
The basic idea is that a user of the browser can override the font size by default. For example, he sees the bad and do my letters larger, or has a 4K monitor.
If you put a base font size in pixels, this setting the user is not running and the letters will be too small. On the other hand have not met with users who change the font size instead of using zoom CTRL+mouse wheel.
Read the information about the units rem and em CSS.
rem em know superficially. When it is necessary to impose, then try them and use. And be sure to always use? Or is it on SEO is not affected? What I hear: SS loyal to the websites mobile version sharpened. - Faustino_Lang commented on July 12th 19 at 16:54
: meta name="viewport" content="width=device-width, initial-scale=1" - Faustino_Lang commented on July 12th 19 at 16:57
: The impact on SEO is - behavioral factors - if the font is too small - the user would take down the website. The mass of this phenomenon will increase the bounce rate. - kelley commented on July 12th 19 at 17:00
: the article was relevant for IE6-7 that were not able to scale the font if it was in pixels - Adelia.Strosin commented on July 12th 19 at 17:03
July 12th 19 at 16:53
This rule has long been obsolete and lost relevance.
His legs grow from about 6-7 times of the Explorer, when the user in the browser settings to change the font size (though 99.9% of users this is still not bother, but not talking about it). It is the font size were changed separately from the other elements of the page.
Today this approach is thrown into the dustbin of history, browsers scale the page (or the parameters of the viewport as view) as a whole. And the font size specified in pixels or anything, this does not hurt. If the user wants to increase - it will increase without any problems. Because in modern browsers, the pixels are no longer tied strictly to the physical pixels of the screen, this logical unit.
In General, we can score.
P. S. of Course, this does not negate the fact that the initial size of the font selected should be reasonable. And for different your devices, using media queries.

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