One more link to further clarify this issue...
Fonts that are merely slanted are called "oblique
". They basically are identical in shape, just slanted a bit.
True italic fonts are actually drawn differently in the more handwritten script style than their upright counterparts.
So really this whole discussion could be characterized better as using normal vs oblique styles with kana/kanji for emphasis borrowing from western writing styles.
I do understand that the oblique kana/kanji do stand out more because the whole character is slanted rather than merely having slightly different writing style. This does make it more effective for emphasis... that's hard to deny.
I just don't think the aesthetic transfers well from roman type to kana/kanji.
Roman type seems utilitarian in nature... simplified and more akin to katakana. Japanese type extends far beyond that into not only scripted writing (hiragana), but full ideographic characters (kanji) which are like entire ideas put into a tiny picture... they're small works of art on their own.
While I can see using an oblique style with katakana and not having it really seem very ugly... it's when you start setting everything, hiragana and kanji as well, all to oblique that things start to look really bad in my opinion.
If, for instance, Japanese were almost entirely written in katakana (in the same way English is written in the Roman alphabet), it would be much easier to use either oblique (slanted katakana), or italic (hiragana) for emphasis and I think avoid having it look as bad because it wouldn't be wrecking the traditional and natural artistic beauty and balance of the scripted and ideographic characters.
I did some tests...
Lines 1, 2, 5, and 6 are all written mainly in katakana. 3 and 4 are written in the "normal" mix of kana/kanji.
On line 1 I tried oblique katakana for emphasis... it doesn't stand out much.
On line 2 I tried normal hiragana for emphasis... it doesn't stand out much either.
On lines 3/4 I tried the normal kana/kanji mix as both normal and oblique... while my non-native eye doesn't notice the change much on the kana, the kanji are much more easily noticeable as being slanted.
But the overall effect is a bad one and doesn't seem to work well for general emphasis.
On line 5 I tried a script font as katakana for emphasis in contrast with the block type of the rest of the kana. This stood out a bit better...
On line 6 I tried a script font as hiragana for emphasis in contrast with the block type of the rest of the kana... this seemed to have the best contrast of all.
Sorry for the length of my post... I was kind of just "thinking out loud" so to speak and enjoying exploring the ideas.
I just find the topic fascinating and can't help it.
It's fun for me to explore the ideas about how italic and oblique texts developed in western cultures using the Roman alphabet, and how that translates to Asian writing systems, and if there might not be better "translations" of these underlying ideas into a native counterpart that conveys the same utility without sacrificing the actual different history, development, and aesthetic of the Asian scripts.
Obviously switching to all katakana block type with hiragana script for emphasis is not a valid option... but it at least illustrated, in my mind, what a better direct "translation" of the emphasis idea from roman → kana would be. Next I'd have to extend that to fix normal Japanese writing... perhaps the clear font change, to a lighter or heavier weight, and between script and block for contrast, is the most optimal and artistically pleasing option?
I tried out some variations, and while line 3 clearly stands out the most, it wouldn't work well for web pages because the default link indicator is the underline... so I subsequently give up. I'm too far out of my element and out of ideas for the moment.
Enough rambling (for now).