<Swing Dev> RFR: 8231286: HTML font size too large with high-DPI scaling and W3C_LENGTH_UNITS [v6]

[Matthias Perktold]

> This PR supersedes #2223.
> The original PR was created from master, whereas this PR is created from a fresh branch.
> 
> Below is a copy of the original description, but as @mrserb correctly [pointed out](https://github.com/openjdk/jdk/pull/2223#issuecomment-767048399), there is actually no relevant change from CSS 2.1 to CSS 2.2, so I <strike>striked out</strike> that part.
> 
> ### Original Description
> 
> @prsadhuk asked me to take over his pull request #1628, since I filed the bug an suggested this fix.
> 
> I thought the current behavior would be buggy, but actually the units are quite precise. I checked the size of a text in font-size of 1 in, and it really was approximately 1 inch. The problem is just that browsers behave differently.
> 
> <strike>
> Swing follows the CSS 2.1 spec: https://www.w3.org/TR/CSS21/syndata.html#length-units.
> But in CSS 2.2, length units where redefined: https://www.w3.org/TR/CSS22/syndata.html#length-units.
> Now px is also an absolute unit, and there are constant conversion factors between all absolute units.
> 
> The CSS 2.2 spec includes the following two notes regarding this change:
> 
>     Note that if the anchor unit is the pixel unit, the physical units might not match their physical measurements. Alternatively if the anchor unit is a physical unit, the pixel unit might not map to a whole number of device pixels.
> 
>     Note that this definition of the pixel unit and the physical units differs from previous versions of CSS. In particular, in previous versions of CSS the pixel unit and the physical units were not related by a fixed ratio: the physical units were always tied to their physical measurements while the pixel unit would vary to most closely match the reference pixel. (This change was made because too much existing content relies on the assumption of 96dpi, and breaking that assumption breaks the content.)
> 
> So the spec changed the behavior to better support high-DPI devices with existing content, and that is exactly my intention with this PR as well.
> </strike>

Matthias Perktold has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 20 additional commits since the last revision:

 - Merge branch 'master' into JDK-8231286
 - Merge remote-tracking branch 'upstream/master'
 - Remove unnecessary import
 - Remove unnecessary JFrame creation
 - Calculate conversion factors rather than hardcoding them for better precision
 - Link to CSS 2.2 spec from length mapping comments
 - Moved HtmlFontSizeTest into ./CSS/8231286
 - Updated copyright year of CSS.java
 - Aligned length mapping comments with space after `//`
 - Move StyleSheet rule into body tag
 - ... and 10 more: https://git.openjdk.java.net/jdk/compare/df416425...1040de5c

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2256/files
  - new: https://git.openjdk.java.net/jdk/pull/2256/files/db90d0a7..1040de5c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2256&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2256&range=04-05

  Stats: 119128 lines in 2976 files changed: 46329 ins; 42875 del; 29924 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2256.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2256/head:pull/2256

PR: https://git.openjdk.java.net/jdk/pull/2256