fix(gatsby-remark-images): enable creating img tag with empty alt attribute

[motou]

Description

add a keyword GATSBY_EMPTY_ALT to enable that an img tag with an empty alt attribute can be created

Documentation

add the keyword description @gatsbyjs/documentation

Related Issues

Fixes #20179

[pieh]

The _SKIP_ seems pretty sensible as a keyword, but I wonder if it shouldn't be both more descriptive and less likely to accidentally hit the reverse situation (someone tries to actually have that as alt and then tries to figure out why instead of that alt it's empty string).

My proposal would be something like GATSBY_EMPTY_ALT - this feature is gatsby specific, and hopefully whoever hit that case would at least be able to search for it easier than just _SKIP_?

Other than that - do you think you could add some tests for this behaviour?

I realize we don't really have alt specific tests - we do have accidental testing of this due to snapshots, but it is accidental. I think something like structure below (don't need to add snapshots)

describe(`image alt attribute`, () => {
  it(`alt tag`, async () => {
    const imagePath = `images/my-image.jpeg`
    const content = `![testing-if-alt-is-correct](./${imagePath} "some title")`

    const nodes = await plugin(createPluginOptions(content, imagePath))
    expect(nodes.length).toBe(1)

    const node = nodes.pop()
    const $ = cheerio.load(node.value)
    expect($(`img`).attr(`alt`)).toEqual(`testing-if-alt-is-correct`)
  })
})

for few scenarios we already have:

• alt tag is specified -> make sure that markup is using that
• alt tag is not specified -> check if filename is used
  and one for using the keyword for "empty alt" -> check if it's actually empty

that can be added to https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-remark-images/src/__tests__/index.js will go long way in assuring we don't regress in future

[motou]

Hi @pieh thanks for reviewing and providing feedbacks! I like the idea of the gatsby special tag and will adopt them in the PR. Tests will be not ignored ;)

[pieh]

This is perfect, thank you!

[motou]

Cool, thanks for merging. When is the new version 3.3.33 available? ;)

[tw15egan]

Thanks, everyone for fixing this upstream so quickly 🙏 ❤️

[pieh]

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