This is an osm2pgsql "style.lua" that incorporates "designation-style" but also does a couple of other changes to aid readability.
The rules in the new "style.lua" are used in place of osm2pgsql's native processing. See the answer to this help question and and osm2pgsql's lua README.
It's designed to work with https://github.com/SomeoneElseOSM/openstreetmap-carto-AJT which is based on a copy of https://github.com/gravitystorm/openstreetmap-carto from Summer 2014 (before a number of problematical changes were made that prevented useful features showing up).
See the readme for designation-style (https://github.com/SomeoneElseOSM/designation-style) for more details about the changes there.
The extra changes here generally fall into one of the following categories:
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Remove some features I'm not interested in to remove clutter (e.g. admin boundaries)
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Remove some features that are misused by some mappers for "colouring in" landuse (e.g. leisure=common). Obviously it's a shame that this removes real commons from the map - suggestions of better solutions here are welcome. "highway=living_street" is also misused - they're displayed as residential.
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Remove road and path names and refs that are not signed on the ground. This is another "colouring in" problem.
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Display many more names of things that are part of industrial, commercial and other areas.
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Display abandoned and dismantled railways. Abandoned railways were removed from OSM-carto because the tag was being misused in some places; they're often major features and really should be shown. Dismantled railways don't really belong on a general-purpose map, but they can be useful if you're trying to make sense of land features, so I've added them too.
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Display proposed railways. This is mostly to help me identify where the HS2 will go locally - it will be of little benefit elsewhere.
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Display historic canals. If a "waterway=canal" is like a "railway=rail", then the equivalent to "railway=disused" is "waterway=derelict_canal". That gets rendered by OSM-carto (but in a barely visible light blue). What gets used for the canal equivalent of "railway=abandoned" and "railway=dismantled" tends to be "historic=canal". In the case of "abandoned-equivalent" canals, they're often still major physical features (but with no water), so they're rendered as per "derelict_canal".
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Display some widely-used tags that have been "deprecated" by OSM-carto, like tourism=bed_and_breakfast.
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Fix some display issues that have since been fixed in OSM-carto. One example of this was that supermarket buildings used to display in pink over the top of pink retail landuse. OSM-carto has since fixed this by displaying all buildings apart from churches in a light grey; I just display supermarkets as normal buildings. Some "new features" are also supported (e.g. tree rows as hedges).
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Display some deprecated tags as "best guess" (e.g. highway=byway as highway=track).
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Handle some mappers' "special" tags. Where it's obvious what something means (e.g. "access:foot" obviously means "foot") just change it.
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Where tags collide, manually choose one of the pair to render (e.g. "amenity=pub" and "tourism=hotel" - I'm far more likely to be looking for a pub than a hotel, so show that symbol).
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For barriers that are essentially either stiles or gates, display as either stile or gate.
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Consolidated shops, offices and leisure facilities so that most or all will get rendered (by openstreetmap-carto-AJT)
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Created new road classes "secondary_sidewalk" and "tertiary_sidewalk" that get rendered with a wider casing by the "sidewalk" branch of openstreetmap-carto-AJT,
A map based on this style is here.
There's a script that gets the latest extract for an area here. That gets two areas - by default Great Britain (which has extra name processing performed on it) and Ireland and Northern Ireland (which doesn't). The two files are then combined.