Generally the font size of H1 tag should be greater than H5 tag, isn't it.

Yes, by default, but as you noted, you can override it with CSS. As far as search engines are concerned, an <h1> tag is just and <h1> tag regardless of CSS. However, this seems really dumb to make the <h1> tag so small for human use. Is it black hat? Kind of, IMHO, since it would be difficult for a human to read. Would I do it? No.

Marshall

I wanted to preserve what I thought was correct usage, i.e., always begin with an h1 and work down the page in sequence with subsequent h tags.

Thats not black hat, just another interpretation of good html. (IMHO)

I still have this feeling that if someone (bots cant read CSS files I guess) from Google visits the page and sees that H1 tags are displayed in a normal font size just for SEO purposes, it will be considered as a wrong manipulation of HTML header tags and not enough credits will be awarded. Or is it?

Say if I write an article inside H4 tags will it be a good approach.

Please suggest.

Say if I write an article inside H4 tags will it be a good approach.

IMHO - Bad idea! Regardless of it's font size, <h> still stands for heading and a heading (normally) would contain only a few words.

You might find this post [webmasterworld.com] interesting.

There is also the accessibility issue from this post [webmasterworld.com].

Braille readers and speech synthesizers emphasize things depending on the tag. As such, <b> has a different connotation than <strong>. They see <b> as a bolded word and <strong> as an important word.

With the <H> tag, they should follow a specific order within the category they cover. Say your category has header section and four subsections and your first <H> tag is <H2>. The next for the subsections should be <H3> or something smaller: <H4>, <H5> or <H6>. All the subsections can be the same <H> tag, but you shouldn't go <H3> then <H4> then back to <H3> because it will interpret the <H4> as a new section. Make sense? Since I tend to give visual examples - I use to be a teacher among other things - it goes like this:
TOPIC-
<H2>
<H3>
<H3>
<H4>
NEW TOPIC -
<H2>
<H3>
<H4>
<H5>
and so on....

WAI also emphasizes not to use <H> tags merely to bold things.

And last, from this post [webmasterworld.com]

If you look at the tags from a "human" perspective, you could compare them to a book. An <h1> would represent the title of a chapter while <h2>-<h6> represents the sub-chapters as in a Table Of Contents.
If you are looking for something specific, you first go to the chapter, but it is the sub-chapter where you will find exactly what you want. Does this make an <h1> tag less important? I cannot say for sure, but IMHO, if sny SE looks at a page as a human would, I believe the 2-6 tags are more specific to search and finding what you want.

Marshall