While this article is a bit of a mess in many ways (structure, content, design), and I wouldn't take it seriously as a guide to design, you can use leading to give emphasis to things like an intro paragraph or pull quote, in which case 150% is totally fine and you might even go over that. They should probably have mentioned the context in which their advice might have been used, and given examples of different usage. 150% on main copy would usually be way too much of course. Even for main text, personally I find 12/14 to be pretty tight - Indesign for example chooses 12/14.4 as a default, and how tight it feels depends on a lot of factors (face, line-length, weight, density of text (is it head, pull-quote, long body?)). There's a lot of variables there and I'd be careful about making sweeping statements about leading.
I agree the kerning examples are pretty absurd, if anything automatic kerning usually gives you too much space - I've never seen automatic kerning make such a mess of spacing as in their example, and as you say it's something you'd only do manually on heads (and even then rarely in web typography as the tools just are not there to do it well, unless you want to litter your code with manual spans).
The practical typography link you gave is great - that's a much better resource for online typography, and beautifully done:
> 150% on main copy would usually be way too much of course.
I think this is true in general, but fonts that are typically used in web design tend to have pretty tall x-heights (designed that way to compensate for low monitor pixel density). As the x-height goes up, I find the leading can too.
I like the rule of thumb that the line spacing (i.e. distance between baselines of successive lines) should be about 3x the x-height.
But I also do tend to like my lines a bit more spaced out than most. Some sort of weird compensation for suffering through years of painful single-spaced Word documents and old web pages.
I agree the kerning examples are pretty absurd, if anything automatic kerning usually gives you too much space - I've never seen automatic kerning make such a mess of spacing as in their example, and as you say it's something you'd only do manually on heads (and even then rarely in web typography as the tools just are not there to do it well, unless you want to litter your code with manual spans).
The practical typography link you gave is great - that's a much better resource for online typography, and beautifully done:
http://practicaltypography.com/
PS Gill tried to shag sheep and his typography was really rather good.