Richard Webb: "Einstein’s special relativity, which he formulated in his “miracle year” of 1905, was a theory that revolutionised our ideas of space and time – and ultimately paved the way for some even bigger surprises. Its origins stretched back half a century. In the 1860s, the Scottish physicist James Clerk Maxwell succeeded in melding electricity and magnetism into one unified theory of electromagnetism. But his equations turned up a surprise. However Maxwell sliced it, the numbers only made sense if light travelled through space at the same, constant speed, regardless of how fast the source of the light itself is travelling. This goes against established classical (often called “Galilean”) ideas of the relativity of speed. If someone fires a bullet from a moving car, for instance, to a bystander the bullet travels at the sum of its speed and the car’s speed. Yet when, 20 years after Maxwell, US physicists Albert Michelson and Edward Morley were looking for the luminiferous ether, a medium supposed to carry light, they too concluded that the speed of light must be a constant, unaffected for example by the Earth’s rotation. Einstein’s genius was to raise the constant speed of light to a principle of nature..." https://www.newscientist.com/term/special-relativity/

Independence from the speed of the light source was a tenet of the ether theory (on which Maxwell's model was based). Combined with the principle of relativity, this independence entails a nonsensical conclusion (Einstein had to wrestle with his conscience "over a lengthy period of time, to the point of despair" before introducing it):

John Stachel: "But this seems to be nonsense. How can it happen that the speed of light relative to an observer cannot be increased or decreased if that observer moves towards or away from a light beam? Einstein states that he wrestled with this problem over a lengthy period of time, to the point of despair." http://www.aip.org/history/exhibits/...relativity.htm

The speed of light CAN be increased or decreased if the observer moves towards or away from a light beam:

Stationary light source, moving observer (receiver): http://www.einstein-online.info/imag...ector_blue.gif

The speed of the light pulses as measured by the source is

c = df

where d is the distance between the pulses and f is the frequency measured by the source. The speed of the pulses as measured by the observer is

c'= df' c

where f' f is the frequency measured by the observer.

In the quotation below Banesh Hoffmann clearly explains that, "without recourse to contracting lengths, local time, or Lorentz transformations" (as was the situation in 1887), the Michelson-Morley experiment is compatible with Newton's variable speed of light, c'=c±v, and incompatible with the constant (independent of the speed of the emitter) speed of light, c'=c, posited by the ether theory and adopted by Einstein:

Banesh Hoffmann, Relativity and Its Roots, p.92: "Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether. If it was so obvious, though, why did he need to state it as a principle? Because, having taken from the idea of light waves in the ether the one aspect that he needed, he declared early in his paper, to quote his own words, that "the introduction of a 'luminiferous ether' will prove to be superfluous." https://www.amazon.com/Relativity-It.../dp/0486406768

Wikipedia: Newton's variable speed of light, c'=c ± v, explains the result of the Michelson-Morley experiment:

"Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment of 1887. [...] The name most often associated with emission theory is Isaac Newton. In his corpuscular theory Newton visualized light "corpuscles" being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v)." https://en.wikipedia.org/wiki/Emission_theory

John Norton: "The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE." http://philsci-archive.pitt.edu/1743/2/Norton.pdf

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