# Fractional MIME types? what about style/class attribute values?

**URL:** <https://discourse.julialang.org/t/fractional-mime-types-what-about-style-class-attribute-values/51263>\
**Category:** General Usage\
**Created:** [December 4, 2020, 4:45pm UTC](https://discourse.julialang.org/t/fractional-mime-types-what-about-style-class-attribute-values/51263 "2020-12-04T16:45:49Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![cce](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cce/32/460_2.png) [@cce](https://discourse.julialang.org/u/cce)\
**Post date:** [December 4, 2020, 4:45pm UTC](https://discourse.julialang.org/t/fractional-mime-types-what-about-style-class-attribute-values/51263/1 "2020-12-04T16:45:49Z")

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Hello. I’m finalizing the implementation of an HTML generation library, [HypertextLiteral.jl](https://github.com/clarkevans/HypertextLiteral.jl) based upon [@mbostock](https://github.com/mbostock)’s [Hypertext Literal (HTM)](https://observablehq.com/@observablehq/htl). The goal of this is to provide server-side HTML generation, using Julia interpolation syntax. I’m wondering if I should be abusing the MIME type mechanism for `style`, `onclick` and other attributes.

Let’s start with some background. This library successfully uses `showable("text/html", obj)` to see if a user defined object can be serialized directly as HTML. If so, then I can directly call `show(io, MIME("text/html"), obj)`. Else, I’m throwing an error asking that applications either: expressly convert the object to a string, or provide their own implementation of `show` for the `text/html` mimetype. I think this is a clean way to enable extensions… for not just this HTML generation library, but any other tool that needs a `text/html` representation of an object. This would, for example, let `HTML` objects be transparently treated without any dependencies.

Along these lines of thinking, for the `<script>` tag, which has its own (lack of) escaping, I think I’m going to follow the same approach, only using `MIME("application/javascript")`. Similarly, for the `<style>` tag, I could ask the user object to serialize to `MIME("text/css")`. I’m assuming any `String` value provided works… this is for custom objects that are not already converted to string values.

Now for my question. What do I do for references to objects that occur within a `style` attribute? I was thinking about using `style = MIME{Tuple{Symbol("text/html"), Symbol("@style")}}`. This convention would decouple the `show` dispatch from our code. Then, I could use `showable(style, obj)` and `show(io, style, obj)` to ask third party libraries to provide a customized adequate serialization to the “`style`” attribute. Thoughts?

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**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [December 6, 2020, 10:29am UTC](https://discourse.julialang.org/t/fractional-mime-types-what-about-style-class-attribute-values/51263/2 "2020-12-06T10:29:11Z")

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> [@cce](#):
>
> I’m wondering if I should be abusing the MIME type mechanism for `style` , `onclick` and other attributes.

Possibly not — that mechanism was introduced for `show` & friends.

I would just define my own functions in the API, which fall back to

```julia
show(io, MIME"text/html"(), obj)

```

unless the user defines a specific method. But there may be cases where the user would want different outputs for `show` and HTML generation. Eg similarly for PGFPlotsX.jl, we have a `print_tex`.

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<div class="post-metadata">

**Author:** ![cce](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cce/32/460_2.png) [@cce](https://discourse.julialang.org/u/cce)\
**Post date:** [December 6, 2020, 2:12pm UTC](https://discourse.julialang.org/t/fractional-mime-types-what-about-style-class-attribute-values/51263/3 "2020-12-06T14:12:15Z")

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Rather than overload MIME, I think we’ll try something that [mirrors](https://github.com/clarkevans/HypertextLiteral.jl/blob/7089ba0fdfa0b60fc150e552c239bbe579cbb1d6/src/HypertextLiteral.jl#L209) how MIME works. I think with more work in the implementation, this could all be specified at macro expansion time so that the result is type stable and the resulting code is perhaps even fast.

```julia
import HypertextLiteral: HTLAttribute, htl_escape

struct Custom data::String end

Base.show(io::IO, at::HTLAttribute{Symbol("data-custom")}, value::Custom) =
    print(io, htl_escape(value.data))

@print @htl("<tag data-custom=$(Custom("a&b"))/>")
#-> <tag data-custom=a&#38;b/>

@print @htl("<tag $(:dataCustom => Custom("a&b"))/>")
#-> <tag data-custom=a&#38;b/>

```

The issue is that within an attribute, especially unquoted attributes, the escaping rules are different, it’s not really `"text/html"` anymore, it’s a horse of a different colour.