Web Fonts and providing fallback fonts

You are presumably using font files and a CSS file as generated by FontSquirrel. The problem with their approach is that each specific font (such as Open Sans Regular and Open Sans Bold) is represented as a separate font-family, with font weight set to normal. This means that instead of markup like <p>foo <strong>bar</strong> and simple CSS like p { font-family: Open Sans, Arial } (letting browsers default strong to bold font weight and select the suitable font from the Open Sans family), you will be forced to set fonts explicitly. This means setting both font family and font weight, implicitly with the font-family property value.

You would need to tune the CSS to get a better approach. You would use the same font family but different weights in the @font-family rule, and in the font-family rule, you would only set the family:

@font-face {
    font-family: 'open_sans';
    src: url('opensans-bold-webfont.eot');
    src: url('opensans-bold-webfont.eot?#iefix') format('embedded-opentype'),
         url('opensans-bold-webfont.woff') format('woff'),
         url('opensans-bold-webfont.ttf') format('truetype'),
         url('opensans-bold-webfont.svg#OpenSans') format('svg');
    font-weight: bold;
    font-style: normal;
}   
@font-face {
    font-family: 'open_sans';
    src: url('opensans-regular-webfont.eot');
    src: url('opensans-regular-webfont.eot?#iefix') format('embedded-opentype'),
         url('opensans-regular-webfont.woff') format('woff'),
         url('opensans-regular-webfont.ttf') format('truetype'),
         url('opensans-regular-webfont.svg#OpenSans') format('svg');
    font-weight: normal;
    font-style: normal;
}
* { font-family: open_sans, Arial; }

And then you would just use font-weight: bold (or HTML markup that has such an effect by default, like strong, b, th, h1 through h6) for those elements that should appear in Open Sans Bold.

Doing it the way you describe appears to work on most browsers, too, but I wouldn’t count on it. Once you have declared a font as normal weight in your @font-face, setting font-weight: bold on text in that font could cause a) a failure, since a the weights don’t match or b) the font taken as a starting point for algorithmic bolding, resulting in double bolding. And if I’m not mistaken, b) is what happens on Safari (Win 7).

You've correctly highlighted an issue with the 'new age' of web fonts, this blog post discusses it and presents a workaround http://elliotjaystocks.com/blog/font-weight-in-the-age-of-web-fonts/

Relevant snippet

Problem number two is significantly bigger than the first. Consider FF Enzo, which doesn’t have a 400 (or even 500) weight. In some circumstances, its Light (300) weight might perhaps be a little too thin for small body type, so let’s use the 600 weight instead. Ah, that looks okay.

But it’s not okay! Because if that font can’t be displayed and we fallback to something else, that fallback font will render at its 600 weight; in other words: bold.

A workaround?

There’s a way around this and it’s the method FontsLive use in the CSS they generate for their users: you declare each weight individually rather than the entire family. Their code looks a bit like this:

CSS code:

{ font-family: 'FamilyName Light', 'FallbackFamily', serif; font-weight: normal; }
{ font-family: 'FamilyName Medium', 'FallbackFamily', serif; font-weight: normal; }
{ font-family: 'FamilyName Bold', 'FallbackFamily', serif; font-weight: bold; }
{ font-family: 'FamilyName Black', 'FallbackFamily', serif; font-weight: bold; }

Update:

@font-face {
    font-family: 'OpenSansBold';
    src: url('../fonts/OpenSans-Bold-webfont.eot');
    src: url('../fonts/OpenSans-Bold-webfont.eot?#iefix') format('embedded-opentype'),
         url('../fonts/OpenSans-Bold-webfont.woff') format('woff'),
         url('../fonts/OpenSans-Bold-webfont.ttf') format('truetype'),
         url('../fonts/OpenSans-Bold-webfont.svg#OpenSansBold') format('svg');
    font-weight: bold;
    font-style: normal;
}

And then something like (as you suggested):

{ font-family: OpenSansBold, 'Arial'; font-weight: bold; }

While the accepted answer works, if you want to provide a really accurate fallback font you will want to define a separate line height and letter spacing for each fallback as well. The main reason to do this is that Google recently introduced a score on Cumulative Layout Shift (CLS), indicating how much the page jumps around upon loading.

You can achieve this by putting a CSS class on the body before the font loading, that gets removed if font loading completes successfully. I do that by including this near the top of the page:

<script>!function(d){if (d.fonts.ready){d.body.className="loading";d.fonts.ready.then(function(){d.body.className=""})}}(document)</script>

You CSS would then look something like this:

h1,p {font-family: Open Sans, Arial}
h1.loading {font-family: Arial;line-height:16px}
p.loading {font-family: Arial;line-height:12px}

By adding and removing the "loading" class in the inspector you can then play with the CSS to get it to stop jumping around. With this you can reliably get the CLS to zero to please the Google gods. The only alternative alternative I've found is using a font loader library which will give a penalty on loading time.